?? You are told exactly which rate- "How fast is the angle of elevation from the observer to the helicoper increasing". Draw a picture- a right triangle having the "observer" at one vertex, the helicopter at another, and the right angle directly beneath the helicopter. The "angle of elevation" is the angle of that triangle at the observer's vertex. You want to find . Note that both the height of the helicopter (the "far side" of the right triangle) and the distance to the observer (the "near side" of the right triangle) are functions of t.