Irodov Problem 1.166

In a perfectly inelastic collision the two masses will fuse and start to move together. Since there are no external forces acting on the system of particles the momentum of the system must be conserved. Let the masses be m1 and m2 and their corrosponding velocities prior to collision be v1 and v2. Let v be the final velocity of the particle. Then we have,
m1v1 + m2v2 = (m1+m2) v.

So in this problem the final velocity will be [3i -2j + 2(4j -6k) ]/(2+1) = i + 2j - 4k