[ms_access] Field that does not show up

[CapitalLease]

I have a record in which I take Field "A" and subtact Field "B" and 
that creates the answer in Field "C". That works fine in the 
record/form, but when I then do a mail merge with a form letter the 
Field "C" sum does not show up in the letter, it reflects a blank. How 
can I get the result of function to show up in a merge mail form?