Calculation in BI publisher template assistance required

Hi all,

We are trying to calculate fields to get balance in the report but still it does not work .Could you help  us in achieving the result using variables ,how,where to declare them and get them..the proper syntax etc..

for eg: This is one of the syntax we tried...

<?xdoxslt:set_variable($_XDOCTX, 'LVar',xdoxslt:get_variable($_XDOCTX, 'a')- xdoxslt:get_variable($_XDOCTX, 'b')+xdoxslt:get_variable($_XDOCTX, 'c') ) ?>

where a - BAL_AMT   ,  b - PYMNT_AMT, c- INVOICE_AMT

but we get different kind of errors like Cannot convert to number, Error in extension function arguments