A body of mass 3 kg moving with a velocity of 4 m/s towards left collides head on with a body of mass 4 kg moving in opposite direction with a velocity of 3 m/s. After the collision the two bodies stick together and move with a common velocity which is : 

zero

Initial momentum : 3 x 4 + 4 (-3) = 0

Final momentum will also be zero. But both stick together, hence both come to rest.