diff --git a/api/controller.js b/api/controller.js
index 949731c..a9efaf1 100644
--- a/api/controller.js
+++ b/api/controller.js
@@ -16,6 +16,7 @@ exports.calculate = function(req, res) {
     'subtract': function(a, b) { return a - b },
     'multiply': function(a, b) { return a * b },
     'divide':   function(a, b) { return a / b },
+    'power':    function(a, b) { return Math.pow(a, b) },
   };
 
   if (!req.query.operation) {
diff --git a/public/client.js b/public/client.js
index 1c60f86..1227a95 100644
--- a/public/client.js
+++ b/public/client.js
@@ -33,6 +33,9 @@ function calculate(operand1, operand2, operation) {
         case '/':
             uri += "?operation=divide";
             break;
+        case '^':
+            uri += "?operation=power";
+            break;
         default:
             setError();
             return;
diff --git a/public/default.css b/public/default.css
index c652a2b..59082df 100644
--- a/public/default.css
+++ b/public/default.css
@@ -41,12 +41,8 @@ BODY {
     left: 0;
 }
 
-#buttons {
-    position: relative;
-}
-
 #result {
-    background-color: #9b9b9b;
+    background-color: skyblue; /* Hot Pink */
     width: 226px;
     height: 63px;
     margin: 0;
@@ -59,18 +55,13 @@ BODY {
     font-size: 25px;
     text-align: right;
 }
-
-/*BUTTON {
-    background-color: #d5d5d5;
-    border: 0;
-    border-radius: 0;
-    width: 55px;
-    height: 42px;
-    position: absolute;
-
-    font-size: 22px;
-}*/
-
+.btn {
+    font-size: 1em;
+    height: 45px;
+    border: 1px solid black;
+    background-color: skyblue; /* Deep Pink */
+    margin: 4px;
+}
 #sign,
 #clear {
     font-size: 17px;
@@ -92,53 +83,6 @@ BODY {
     display: inline-block;
     width: 25px;
     height: 45px;
-    background: url('digits.png') 0 0 no-repeat;
-    background-size: cover;
-    margin-top: 10px;
-    font: 0/0 Arial;
-    color: rgba(255, 255, 255, 0);
-    text-indent: -9999px;
-    z-index: 1;
-}
-
-.digit0 {
-    background-position: 0 0;
-}
-
-.digit1 {
-    background-position: -25px 0;
-}
-
-.digit2 {
-    background-position: -50px 0;
-}
-
-.digit3 {
-    background-position: -75px 0;
-}
-
-.digit4 {
-    background-position: -100px 0;
-}
-
-.digit5 {
-    background-position: -125px 0;
-}
-
-.digit6 {
-    background-position: -150px 0;
-}
-
-.digit7 {
-    background-position: -175px 0;
-}
-
-.digit8 {
-    background-position: -200px 0;
-}
-
-.digit9 {
-    background-position: -225px 0;
 }
 
 .resultchar.decimal {
@@ -176,11 +120,3 @@ BODY {
 .calculator {
     max-width: 320px;
 }
-
-.btn {
-    font-size: 1em;
-    height: 45px;
-    border: 1px solid black;
-    background-color: #d5d5d5;
-    margin: 4px;
-}
diff --git a/public/index.html b/public/index.html
index 400c454..1fd7a8d 100644
--- a/public/index.html
+++ b/public/index.html
@@ -41,6 +41,8 @@
             <button class="btn" onClick="equalPressed()">=</button>
 
             <!-- TODO: Buttons -->
+            <!-- add a button for a power (or exponential) function -->
+            <button class="btn" onClick="operationPressed('^')">^</button>
           </div>
         </div>
       </div>
diff --git a/test/arithmetic.test.js b/test/arithmetic.test.js
index deded48..bc3441c 100644
--- a/test/arithmetic.test.js
+++ b/test/arithmetic.test.js
@@ -94,6 +94,57 @@ describe('Arithmetic', function () {
     });
 
 // TODO: Challenge #1
+// add tests for subtraction
+    describe('Subtraction', function () {
+        it('subtracts two positive integers', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=42&operand2=21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 21 });
+                    done();
+                });
+        });
+        it('subtracts a positive integer from zero', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=0&operand2=21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: -21 });
+                    done();
+                });
+        });
+        it('subtracts a negative integer from a positive integer', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=42&operand2=-21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 63 });
+                    done();
+                });
+        });
+        it('subtracts two negative integers', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=-42&operand2=-21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: -21 });
+                    done();
+                });
+        });
+        it('subtracts two floating point numbers', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=42.5&operand2=21.5')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 21 });
+                    done();
+                });
+        });
+        it('subtracts with negative exponent', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=1.2e-5&operand2=-1.2e-5')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 0.000024 });
+                    done();
+                });
+        });
+    });
  
 
     describe('Multiplication', function () {