Here is math problem related to amateur astronomy that is well defined and not overly complicated. I came across this problem when thinking about how to write a computer program to plot the path of reflected light rays off mirrored surfaces.

What is the equation relating the slope (i.e. rise/run) of the reflected ray to the slope of the incident ray and the slope of the surface at the point of reflection? To keep the problem manageable just consider two dimensions (i.e. x and y).

No cheating. Looking for the answer in a book or studying the source code of an existing program is forbidden. See a future issue of Astrotent for the answer.