Given $\displaystyle f(x) = 3x -6$, algebraically determine $\displaystyle y = f^-1(x)$ and the invarient point(s).

$\displaystyle y = 3x - 6$

$\displaystyle x = 3y - 6$

$\displaystyle x + 6 = 3y$

$\displaystyle y = \frac{x}{3} + 2$

That is as far as I got. I know the answer is (3,3), but how do i show this.