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