How did they get this answer

Part VII: Calculate the distances. (23 points) 1. What is the distance from the hazard buoy P to the southern landmark L2? Use this formula: d2 - (DL2)"+ (PD)", where d is the distance from P to L2, DL2 is the distance from D to L2, and PD is the distance from P to D. Round to the nearest foot. (5 points) D = 3970 feet. 2. What triangles are similar? How do you know? (5 points) ero.com/file/87787558/272-Math-lldocx/