I'm reading that as the inverse function of x. x^2 - 16 if you subtract 16, from x^2 , then you have to add 16 to get back to x^2 (from which you would then do the subtraction to complete f(x), let us call f(x) y, then if we have y + 16 then we take the square root to get back to original x _________________|/ y` + 16 so x =9 f(x) == 81 -16 ==65 :: y=65y = 65: add 16, get 81, take the s root, that is 9, the original input ta da :)