Stopping distance of a racing car during a test by the

Question

Question: STOPPING DISTANCE OF A RACING CAR During a test by the editors of an auto magazine, the stopping distance s (in feet) of the MacPherson X-2 racing car conformed to the rule

s = f(t) = 120t - 15t²          (t ≥ 0)

where t was the time (in seconds) after the brakes were applied.

a. Find an expression for the car's velocity √ at any time t.

b. What was the car's velocity when the brakes were first applied?

c. What was the car's stopping distance for that particular test?