The Coordinate Plane · Superchallenge problem

Two rectangles with sides along grid lines overlap. The first has corners $(0, 0)$, $(6, 0)$, $(6, 4)$ and $(0, 4)$. The second has corners $(3, 2)$, $(9, 2)$, $(9, 7)$ and $(3, 7)$. What is the perimeter of the whole shape they cover together (its outside edge only), in units?