Let P be a point in the first octant, whose image $Q$ in the plane $x+y=3$ (that is, the line segment $P Q$ is perpendicular to the plane $x+y=3$ and the mid-point of PQ lies in the plane $x+y=3$ ) lies on the z -axis. Let the distance of P from the x -axis be 5 . If R is the image of P in the xy -plane, then the length of PR is $\_\_\_\_$ .