When they ask you to evaluate and give you f, what they actually mean (and what inverses mean) is what value of x do you plug into f to get the value y, cuz inverses take the range of f to the domain of f

So what value of x needs to be plugged into f(x) to get 8, that should be obvious (consider easy numbers)

then the second part, you should know that functions and their inverses "undo" eachother, or in other words and