The function g takes some input x from the *domain* and maps it to some value g(x) in the *range*. The function g^-1 takes some value g(x) from the *range* and maps it back to its associated value x in the *domain*. Essentially, when you feed an input through a function, and then feed that output through its inverse function, you get the original input back. So your answer here would be D. 13, since that was your original input.
