let's recall that for a function f(x) that has an inverse function f⁻¹(x), the domain of f(x) corresponds to the range of f⁻¹(x), what the heck does that all mean?
well, it means that for any coordinate pairs (a , b) in f(x), there's a corresponding pair of (b , a) on f⁻¹(x).
we know in this case that f(-4) = 15, or namely that f(x) has a pair of (-4 , 15), well, that means that f⁻¹(x) has a swapped pair of (15 , -4).
the above means that f⁻¹(15) = -4.
