Generating an expiry date based on fields in different tables.

I want a field that gives the date a certificate expires.

I think i would create a query and add a field using something like this..

dateadd("d", [DaysToExpire], [DateCompleted])

My trouble is, [DaysToExpire] is on a different table to [DateCompleted]. how do i resolve that?

Is there a way to get the query field to refer to both tables?

Can I create a lookup field on one of the tables that auto populates once the 2 tables a linked by a record?

I am all pretty new to this so sorry if my language is a bit off. Any help would be much appreciated.