You can do this like this.
function MultiplyTwoNumber()
{
var a=document.getElementById(<controlId>).value;
var b=document.getElementById(<controlId>).value;
var c = parseFloat(a) * parseFloat(b) ;
document.getElementById(<labelcontrolid>).value = c;
}
Call this from button control.
<asp:Button OnClientClick="return MultiplyTwoNumber();" Text="Multiply" runat="server" />