How do calculate values in same field?

First off I found a post that I think is the same or close to what I"m asking but I don't quite understand it...

http://www.experts-exchange.com/Microsoft/Development/MS_Access/Q_21045189.html?sfQueryTermInfo=1+calcul+field+same+valu

OK, my question....I have a field in a table called "ac_balance." It is my beginning balance of money for the month. each record is also associated with a year and a month so I can find the record i need. What I need is to take September's Beginning balance and subtract it from October's beginning balance to see my "vs. Prior Month (a calculated field in a report)"

here's what my sql code looks like;

SELECT [Accounts Receivable].ac_school, [Accounts Receivable].ac_activebalgoal, [Accounts Receivable].ac_vspriormonth, [Accounts Receivable].ac_inactivegoal, [Accounts Receivable].[ac_>15verification], [Accounts Receivable].[ac_<15verification], [Accounts Receivable].ac_entrydate, [Accounts Receivable].ac_balance, [Accounts Receivable].ac_inactive_balance
FROM [Accounts Receivable]
WHERE ((([Accounts Receivable].ac_fiscalyear)="2009") AND (([Accounts Receivable].ac_month)="October"));

It displays most of the info I need but I need, in addition to what the code is giving me already, to display this calculated field value;

here's a short example of what I"m looking for

Accounts Receivable

ac_fiscalyear     ac_month      ac_balance
2009                  October           $100.00
2008                  September       $90.000

I need to find the difference between the two records for ac_balance and I need to display that value as a new field in a query (preferably the same query if at all possible). Because I'm building a report off of this query and will be including that data in the report.