how would I solve this using cal 1 dervitaives

Suppose a police officer is 1/2 mile south of an intersection, driving north towards the intersection at 45 mph. At the same time, another car is 1/2 mile east of the intersection, driving east (away from the intersection) at an unknown speed. The officer's radar gun indicates 30 mph when pointed at the other car (that is, the straight-line distance between the officer and the other car is increasing at a rate of 30 mph). What is the speed of the other car? Speed = mph. Now suppose that the officer's radar gun indicates -30 mph instead (that is, the straight-line distance is decreasing at a rate of 30 mph). What is the speed of