Random guess:
Html:
<script type="text/javascript">
<!--
var1 = <value>;
var2 = <value>;
var3 = var 1 * var 2;
document.all.writetome.innerHTML = var3;
//-->
</script>
<div id="writetome"></div>
Edit ~ I was wrong

I got this to work though...
<form name='writetome'>
<input textarea name='meme' value='hello'>
</form>
<script type='text/javascript'>
<!--
var1 = 5;
var2 = 6;
var3 = var1 * var2;
document.writetome.meme.value= var3;
//-->
</script>