College Algebra (6th Edition)

$f(x) = x + 1$ and $g(x) = x -1$
Let $f(x) = x + 1$ and $g(x) = x -1$: $$f(g(x)) = (x -1) +1 = x$$$$g(f(x)) = (x +1) -1 = x$$ Therefore, $f(g(x)) = g(f(x))$