I'm having a really hard time with this question:

A searchlight rotates at a rate of 3 revolutions per minute. The beam hits a wall located 8 miles away and produces a dot of light that moves horizontally along the wall. How fast (in miles per hour) is this dot moving when the angle between the beam and the line through the searchlight perpendicular to the wall is ?. Note that .

I know what I need to do: relate two variables, differentiate, and solve. But I can't figure out how to relate the given variables (which might say more about my reading comprehension than calculus skills :|). Would anybody mind helping me set it up?