In general you don't choose x = f(x) where f(x) is the function which you want to find the inverse of. That would be confusing it with the original x variable in the expression. Choose y = f(x). Then express x in terms of y. When you're done, change the y in the final expression back to x, and that is your inverse function.
EDIT: By change back to x, I mean replace y with x.