The diagram below shows the graph of a function $f$ along with points $P$ and $R$, which lie on the graph. Point $Q$ is chosen so that $\triangle PQR$ is a right triangle. (Click on the graph to display a larger version.)

Give a formula, in terms of $a$, $f$, and $h$, for the slope of the line through points $P$ and $R$.
Slope = .