Two carts with masses of 4.0 kg and 3.0kg move toward each other on a frictionless track with speed

Momentum = mass * velocity 

Cart 1: 
mass = 4.0 kg 
velocity = 5.0 m/s 
momentum = 20 

Cart 2: 
mass = 3.0 kg 
velocity = -4.0 m/s (opposite direction) 
momenum = -12 

Cart 1 and 2: 
mass = 7.0 kg 
velocity = ?? 

20 - 12 = 7v 
8 = 7v 
v = 8/7 
v = 1 1/7 m/s 

So your answer should be: 1 1/7 m/s in the direction of the heavier