We want to show that f(g(x)) = x and g(f(x)) = x. So to do that, we need to calculate f(g(x)) and g(f(x)) explicitly. Let's begin with f(g(x)) Now, f(g(x)) = f(4/x), by definition of g, as g(x) = 4/x What then is f(4/x) equal to? Use the definition of f.