I want to do a calculation in a datagrid.  I want to subtract values in
columns 1 and 2 to give the answer in column 3. ie;  Column 2 - Column 1 
Column 3

Column        1        2       3
-------------------------------------------
Row 1          2        4       ?
Row 2          7       21      ?


How do I do that?
The datagrid was created using the toolbox.


Any suggestions will be appreciated.
Thanks in advance.