(-х)2n

Interpret as (-x)^{2n}.

If n is an integer, (-x)^{2n} = [(-x)^2]^n = (x^2)^n = x^{2n}.

So the simplified form is x^{2n} (or equivalently (x^2)^n). Example: (-x)^6 = x^6. If you had a different meaning (e.g., -x^{2n} without parentheses), let me know.