Changing a file name in a formula with data validation

Thank you so much for your help.

The correct full file path would be: ='D:\Projects\East\[Sales_16-Washington.xls]Area2'!$O$25

I got the formula to work using =INDIRECT.EXT("'D:\Projects\East\[Sales_"&Criteria!$A$1&"-Washington.xls]Area2'!$O$25")

I'm not using a drop-down for the sheet selection at this point.

I really appreciate your help!

How do I mark this "Solved?"