calculating values

Hi
I'm having a hard time of this... after spending the whole day researching this forum , I managed to get the custom value to do exactly as I pleased..
Today I came back to find it was missing grrrr lol!
Basically what I'm trying to do is setup a new third column that + up the value from column 1 and 2 and displays the result
The custom script was something like this off the top of my head..
global $data;

$data['thismonth'] + $data['lastmonth'];

$data = $value;
This doesn't work <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=7590&image=1&table=forumtopics' class='bbc_emoticon' alt=':(' />
Any help would be very much appreciated ..
Andy