Coastline question math, hard concept

Coastline question math, hard concept

Suppose there’s a straight coastline and a lighthouse that is L=3 miles away from the coast. This light revolves at one revolution per minute. How fast is the beam of light travelling along the coastline? When the beam is 3L away from the coastal point closest to the light, how fast is the light travelling along the coast.

Explain how you got to the answer.