Area of Triangles · Superchallenge problem

Triangle $PQR$ has corners $P = (0, 0)$, $Q = (9, 3)$ and $R = (3, 9)$. Point $S$ is on side $QR$, and the segment $PS$ cuts the triangle into two triangles: $PQS$ has twice the area of $PRS$. If $S = (x, y)$, what is $x + y$?