(a) As the car is moving with constant velocity, it means the rate change of velocity does not change, therefore the average acceleration of the car is zero.
Thus, there is no acceleration, when velocity is constant.
(b) Average acceleration,
Here, v is final velocity and u is the initial velocity and is the time interval.
As twelve seconds later, the car is halfway around the track and traveling in the opposite direction with the same speed, therefore
Thus, the average acceleration of the car is in the direction to the left.