Javascript page not working

Question

<script>
var drop = document.getElementById("dropfuel").value;
var ifuel = document.getElementById("tfuel").value;
function fuelans(drop,ifuel){
return drop * ifuel;
}
document.getElementById("ans").innerHTML = fuelans;
</script>

<p>
<button onsubmit= "fuelans()">Calculate</button>
</p>

<p id="ans">

Cannot get these to work
There the form is as follows:

Dropdown
Input(input*dropdown value)
Button
Line f/ output(output button click)

Input(input*36/10000)
Button
Line f/ Output(output button clicked)