Algebraic Expression Manipulation

Hello:

I posted this to the mathematica newgroup but somehow it never
appeared. So
I am positing it here.

I need a way to take an expression like
(-1)^(2x + 3 y) and be able to reduce it to (-1)^y.

I could not figure out how to do this in Mathematica and need
suggestions
as to how one might go about doing this in other packages (like Maple).

Thanks!

.

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