A helicopter leaves a base, rising straight up at a speed of 15m/s. At the same time that the helicopter leaves, an observer starts from a point 100m away from the base and moves on a straight line away from the base at a speed of 80m/s. How fast is the angel of elevation from the observer to the helicopter increasing when the observer is

a) 400m from the base

b) 600m from the base

Thanks i;m not sure which rates i have to find.