I have two fields named xx and zz another name

I think this line of code and get the total of xx and zz

“groupText” => array (‘> Total XX: {xx} </ b>’, ‘<b> Total ZZ: {zz} </ b>’)

What I want is to see the difference or the result of a subtraction between xx and zz ww

“groupText” => array (‘> Total XX: {xx} </ b>’, ‘<b> Total ZZ: {zz} </ b>’, ‘<b> BALANCE WW: {xx – zz = ww } </ b> ‘)

How to do?