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
