So you should be able to plug both of those into the corresponding equations and they should equal x. This is helpful especially if you are given two equations and asked if one is the inverse of the other.
In order to get inverse functions you can solve for x in the original equation, then exchange x and y, or interchange x and y and then solve. (Whoops, sorry about that, edited it)
What's the cube root of -64 and what's the cube root of -27? Hint, hint.