Velocity and momentum?

1. The problem statement, all variables and given/known data

A 24,000 kg box car travelling north at 2 m/s collides with an 18,000 kg box car. Upon collision the two box cars lock together. What is the velocity of the 18,000 box car before the collision if after the collision the two box cars are traveling south at 1 m/s?

2. Relevant equations

MaVai + MbVBi = MaMaf + MbVbf

3. The attempt at a solution

 

like the cars combined

 

mass of a, velocity of a initial etc