Answer:
The cars would both move in the direction with the greatest momentum. We know the truck would have the greatest momentum due to having the greatest mass. Plus, factors which affects momentum are mass and velocity.
Thus, the car would move forward towards the direction of momentum of the truck. Both will continue to move at the same velocity.
E.g. truck hits the back of a stationary car, both will move forward, to the right & travelling at the same speed, but lower than the initial velocity of the truck. Now the total the total momentum before the collision is the same as the total momentum after the collision; as a result, momentum has been conserved.
Hope this helps!