How long after starting from rest is the car again at rest?

A car is stopped at a traffic light. It then travels along a straight road so that its distance from the light is given by x(t) = bt2 - ct3, where b = 2.40 m/s2 and c = 0.130 m/s3.
(a) Calculate the average velocity of the car for the time interval t = 0 to t = 8.0 s.
(b) Calculate the instantaneous velocity of the car at the following times:
i) t = 0
ii) t = 4.0 s
iii) t = 8.0 s
© How long after starting from rest is the car again at rest?