please help me understand this before my official test

3. (10 points) A drone flying 20 feet per second at an altitude of 60 feet flew directly over an observer. The figure shows the relationship of the drone to the observer at a later time. Drone 60 Observer Find an equation relating r and y and use it to find the value of a when y = 100. Assume that both r and y are functions of time t. How fast is the distance from the observer to the drone changing at the instance of time when the drone is 100 feet from the observer