Any function can be thought of as a set of ordered pairs: {(x, y)}. The corresponding inverse function, if there is one, is the set {(y, x)}. That's why Plato swapped x and y in the given formula, Technically, once you have done that, you have the inverse function. But in order to show that this is a "function", you need to solve for y.
(If I were doing a problem that has, as a hint, "use the two step method", I would start by leafing back through the previous chapters to find out what the "two step method" was.)