I really need help with this hard physics problem below!?

[jpc41]

A man of mass M1 = 104 kg is standing on a M2 = 5420 kg railroad flatcar that rolls without friction to the right with speed 1.9 m/s. Calculate the change in velocity (in m/s) of the flatcar, if the man runs to the left so that his speed relative to the original velocity of the car is 3.9 m/s.

Hint:
Momentum has to be conserved. The man and flatcar have to be moving with the same momentum before and after he starts walking. Before both flatcar and man are going to the right with the same velocity. Afterwards the car is moving to the right but the man is moving to the left with the difference of the two velocities.
[M(car)+M(man)]v(initial) = M(man)[v(initial)-v(man rel to car)] + M(car)v(final)