Answer:
f(g(x)) = x implies that f and g are inverses; g(f(x)) = x also implies that.
Explanation:
Suppose you have two functions f(x) and g(x). If you use g(x) as the input to f(x), and the result is merely x, then we conclude that the two functions are inverses of each other. Similarly with f(x) as argument of g(x).
f(g(x)) = x implies that f and g are inverses; g(f(x)) = x also implies that.