Angularjs Controller and Expression Behaviour

I’m very new to angularjs and going through the tutorials. I’m held up at a point. In the below code why is {{result}} value not being displayed. What is the flow of code in angularjs:

<html>    
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.9/angular.min.js"></script>
<body>

<div ng-app="myApp" ng-controller="myCtrl">
{{ result }}
</div>

<script>
var app = angular.module("myApp", []);
app.controller("myCtrl", function($scope) {
$scope.firstNum = 6;
$scope.lastNum = 7;
$scope.result="firstNum*lastNum";
 });
</script>

</body>
</html>

Here is the code http://plnkr.co/edit/oH0gnKExBo9UHmXPuOPS?p=preview

Source: AngularJS