a.) Examine whether a car is speeding up or slowing down by computing the magnitude of the velocity for each line. If the speed is decreasing, a car will be slowing down. For such a case its velocity-time graph will start away from $v=0$ and approach the $v=0$ line. Hence we conclude that color{blue} boxed{bf mbox{cars 1, 2, and 4}} are slowing down.

b.) For a car to speed up its speed must be increasing. Therefore, we find that car 3 is speeding up as its velocity-time graph begins close to the $v=0$ line and then gets further away.