According to the category you assigned with your question, either you want or you want .
Imagining that you want the inverse function for ;
you can try switching the purposes of x and y, their positions in the equation, and solve for y.
-------the inverse
That is not the correct way to handle the notation for finding function inverse, but the method IS very practical. Better use of notation is to give a function name to your original function, such as f(x), and then call the inverse as f^-1(x). You also use these notations in the algebraic steps performed.