Question

Let f(x) = x2 − 81. Find f−1(x)

Answer 1

This equation states that x equals -1. So you plug in -1 to all the x's and solve. 
So, your equation would be f(-1) = (-1)(2) - 81
So, f(-1) = -2 - 81
Then, this means that f(-1) = -83
So........
f(-1) is equal to -83