Physics question from bmat

Idk how to calculate the net velocity? Is it that I just need to subtract those since they are in opposite directions? or do i need still need to consider about the given volume & density values??

the balls stick together so it is an inelastic collision and we use the formula:
m1v1 + m2v2 = (m1+m2)v’
density = m/v
since the balls have the same volume and density A = 2 * density B, then mA = 2mB
i will use mA = 2m and mB = m
2m * 6 - m * 3 = (m+2m)v’
(notice the sign of the momentum for B is negative because it is moving in the opposite direction to A)
12m - 3m = 3mv’
3=v’ to the right