Forgive me if I explain this poorly, but allow me to try. Basically if you are given a function "f(x)" which has a result of "y", then the inverse function "g(y)" would output "x". A function and its inverse are two functions f and g such that calling f(g(x)) will result in x and the same is true for calling g(f(y)) resulting in y. The inverse function will "undo" the operation of the other fuction. For example, if you take x^2=y (x squared with the result of y) then the inverse function is square root. sqrt(y) = x. So if f(x) is squaring a number and g(x) is taking the square root of a number then calling g(f(x)) is equivalent to squaring a number and then taking the square root of that number ultimately resulting in the number you started with: sqrt(x^2) = x

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And for test case 3, well, that is simply broken like the entire question.

I hope this helps.