calculus AB help with related rates

A hot-air balloon is rising straight up from a level field and is tracked by a range finder 500 ft from lift-off point. the balloon rises at a rate of 15 feet per second.

A) How fast is the direct distance between the range finder and the ballon changing at 5 seconds

B) how fast is the angle of elevation changing when the ballon is 500 feet high

he gave us a diagram of a right trianle with the hypotenus labeled d and the side labled h and the bottom equalling 500

i think i solved part a and i got 2.223 ft but i wasn't sure how to do part b

any help is appreciated