(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 100difthe 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.