index.html

<head>
<script>
function calculateResult (form) {
	var lengthvar = form.length.value;
	var breadthvar = form.breadth.value;
//	alert (lengthvar * breadthvar);
	document.getElementById("finalResult").innerHTML = "Answer : " + lengthvar * breadthvar;
}

function cancelResult (form) {
		document.getElementById("finalResult").innerHTML ="";
}
</script>

</head>
<body>

<form>
  <label for="length">Length:</label><br>
  <input type="number" id="length" name="length" value="1"><br>
  <label for="breadth">Breadth:</label><br>
  <input type="number" id="breadth" name="breadth" value="1">
  <input type="button" name="finalResult" value="Calculate" onClick="calculateResult(this.form)">
  <input type="button" name="Cancel" value="Cancel" onClick="cancelResult(this.form)">
</form>

<p id="finalResult"></p>


</body></html>