How to do this calc problem?

a police cruiser hunting for a suspect pulls over and stops at a point 20 ft away from a straight wall. the flasher on top of the cruiser revolves around at a constant rate of 90 degrees/ second, and the light beam casts a spot of light as it strikes the wall. how fast is the spot of light moving along the wall at a point 30 ft from the point on the wall closest to the cruiser?