As it sounds the inverse function essentially does the reverse of another function.
So you want to rearrange this expression to solve for x.
(y-9) = (x+3)3
(y-9)1/3 = (x+3)3(1/3)
(y-9)1/3 = (x+3)
(y-9)1/3 - 3 = x
so x = (y-9)1/3 - 3 is the inverse function here