(b) Express the distance s between the plane and the radar station as a function of d.
Since the plane has an altitude from the radar station of 1 mile, then you must use the pythagorus theorum:
The answer would be 100d if the plane was on the same altitude as the radar station, but in this case it isn't.
(c) Use composition to express s as a function of t.
Now you know that d = 100t, so you substitute that into equation (b) to express s as a function of t.