I need some help setting up this related rates problem.

I know this problem is about using similar triangles but I'm not quite sure where to start. Can someone just give me a basic equation to start with? I've looked at similar problems but they all do things differently. :|Quote:

A man 6 ft tall is walking at what rate of 3 ft/s toward a streetlight 18 ft high. At what rate is his shadow length changing?