A 1150 kg automobile is at rest at a traffic signal. At the instant the light turns green, the automobile starts to move with a constant acceleration of 3.8 m/s2. At the same instant a 1900 kg truck, traveling at a constant speed of 7.4 m/s, overtakes and passes the automobile. (a) How far is the center of mass of the automobile-truck system from the traffic light at t = 3.9 s? (b) What is the speed of the center of mass of the automobile-truck system then