How do you simplify #(9x)^(a/b)#?
Rearrange fraction exponent into a radical.
When the exponent is a fraction, the denominator is paired with the terms as it's own exponent.
This pairing is under a radical, with the numerator as the index.
As a result, you get something like this:
P.S. There may be other ways to simplify the value you gave us. The radical route was the first one I thought of.
Hope this helps :)