Subtract two columns

Hi.  Thank you for your response.

The results of my latest attempt ar shown here.  This is wrong, of course.  It should show the net amount for each line.  The first line should have 1869.95.  The second should be 1978.06, etc.

I have the two tables, _GroupEarnings and _GroupDeductions, linked by the fields _EmplAndPeriod (ie:  

One thing that bothers me is that the field, _DedAmount, has the 'sum' symbol in front of it, while the field _EarnAmount does not.  I'm not sure where that came from when I created the field, _DedAmount, or why it's there: