Two rocks collide in outer space. Before the collision, one rock had mass 12 kg and velocity ‹ 4450, −3000, 2200 › m/s. The other rock had mass 6 kg and velocity ‹ −600, 2000, 3750 › m/s. A 2 kg chunk of the first rock breaks off and sticks to the second rock. After the collision the 10 kg rock has velocity ‹ 1450, 400, 1900 › m/s. After the collision, what is the velocity of the other rock, whose mass is now 8 kg?
I found the momentum for the first one before and after collision and the second one after the collision. then I used mv+mv=mv+mv. Not sure if the right way to approach it.

Question

Two rocks collide in outer space. Before the collision, one rock had mass 12 kg and velocity ‹ 4450, −3000, 2200 › m/s. The other rock had mass 6 kg and velocity ‹ −600, 2000, 3750 › m/s. A 2 kg chunk of the first rock breaks off and sticks to the second rock. After the collision the 10 kg rock has velocity ‹ 1450, 400, 1900 › m/s. After the collision, what is the velocity of the other rock, whose mass is now 8 kg? 
I found the momentum for the first one before and after collision and the second one after the collision. then I used mv+mv=mv+mv. Not sure if the right way to approach it.