Please help with very simple JavaScript logic.  I want a my xml button to call a javascript function passing parameters then calculate and return an answer for display. Below is the relavant part of my XML file

<field name="x" />
<field name="y" />                
<field name="z" />                
<script language="JavaScript">
     function findWriter() {                        
         //user to enter numeric values x and y
         //this script to calculate z = x + y and update <field name="z" />  
     }
</script>                                              
<input type="button" value="Display Writer" onClick="findWriter()"/>