JavaScript

Membuat rata-rata nilai kuliah dengan javascript

Hello gan ... pingin tau giman caranya buat rata-rat dengan javascript?
ini contoh rata-rata nilai kuliah MTK:
MataKuliahNILAI
KALKULUS
GEOMETRY
KEWAEGANEGARAN
PANCASILA

nah maw tau scriptny gmn ?
ini dibawah gan?

<form name="form">
<table width="60%" border="0"><tbody><tr><td width="50%" valign="top">MataKuliah</td><td width="50%" valign="top">NILAI</td></tr><tr><td width="50%" valign="top">KALKULUS</td><td width="50%" valign="top"><input type="text" name="angka1" size="3" value="70" /></td></tr><tr><td width="50%" valign="top">GEOMETRY</td><td width="50%" valign="top"><input type="text" name="angka2" size="3" value="78"/></td></tr><tr><td width="50%" valign="top">KEWAEGANEGARAN</td><td width="50%" valign="top"><input type="text" name="angka3" size="3" value="80"/></td></tr><tr><td width="50%" valign="top">PANCASILA</td><td width="50%" valign="top"><input type="text" name="angka4" size="3" value="60"/></td></tr></tbody></table>
</form>


<button type="button" onclick="cek()" >RATA2</button>
<h3 id="tampil"></h3> <br>

<script>
function cek(){
var a=form.angka1.value;
var b=form.angka2.value;
var c=form.angka3.value;
var d=(a-(-b)-(-c))/3;
document.getElementById("tampil").innerHTML=d;
}


</script>