A 6,600 kg train car moving at + 2.0m/s

The answer is v = -.2 5

Assuming the whole question is this:

A 6600 kg train car moving at + 2.0 m/s bumps into and locks together with one of mass 5400 kg moving at 3.0 m/s. What is their final velocity?

you would do:

6600 (2) - 5400 (3) = (6600+5400) v

13200 - 16200 = 12000v

v = - 0.25 m/s