Please help stworthy, I want to sum from two to one numberbox in the form..
<fieldset id="fieldset">
<div align="center" class="ftitle">Isi Form Pelanggan Baru</div>
<form id="fm" method="post" novalidate>
<tr>
<td class="fitem">
<input id="tagihan_bln" name="tagihan_bln" class="easyui-numberbox" formatter="rupiah" style="width:250px;height:30px;text-align:right" data-options="label:'Tagihan :',precision:0,groupSeparator:','" required="true"></td>
<td class="fitem">
<input id="potongan" name="potongan" class="easyui-numberbox" formatter="rupiah" style="width:250px;height:30px;text-align:right" data-options="label:'Fee :',precision:0,groupSeparator:','" required="true"></td>
<td class="fitem">
<input id="tagihan_sisa" name="tagihan_sisa" class="easyui-numberbox" formatter="rupiah" style="width:250px;height:30px;text-align:right" data-options="label:'Sisa :',precision:0,groupSeparator:','" onChange="true" readonly="true"></td>
</tr>
</form>
</fieldset>
//code
$('#fm').form('load');
var a = $('#tagihan_bln').numberbox('getValue');
var b = $('#potongan').numberbox('getValue');
var sum = a.numberbox('getValue')-b.numberbox('getValue');
$('#tagihan_sisa').numberbox('setValue',sum);