If r(x) is an inverse of s(x), then we know that r(s(x)) = x and s(r(x)) = x.
Let's think about what inverses are in order to help us understand.
We know that a function, in order to be an inverse of another, has to be reflected about the y = x line.
So, if a function is reflected about the y = x line, then if we have a composite function, they will always result in the same input (ie x)