trying to solve some physics problem in preparation 4 quiz but got stuck

[elizbie]

on this one please help?? Four railroad cars, each of mass 2.40 * 10^4 kg, are coupled together and coasting along horizontal tracks at speed vi toward the south. A very strong but foolish movie actor, riding on the second car, uncouples the front car and gives it a big push, increasing its speed to 4.40 m/s southward. The remaining three cars continue moving south, now at 2.00 m/s.
(a) Find the initial speed of the cars.
m/s

(b) How much work did the actor do?
kJ