I need to averge based on several criteria.

In column C I have ages
In column M I have words for medication used by a patient
In column N I have numbers

I'd like to search column C for certain ages and then M for a drug and then
average the dose (in column N) for those meeting criteria for both colums C
and M

i.e. average dose (column N) for people under 70 (column C) on drug x
(column M).

I'd really appreciate any help.

Paul