A car drives counterclockwise around a horizontal circular track with radius R = 20 m, centered at the origin in the x-y plane. As it drives around the track, its speed steadily increases. At t = 0, the car's velocity is 9 m/s . At t = 3 s, the car's velocity is 15 m/s. What is the average acceleration of the car from t = 0 to t = 3 s, in m/s2 ?