If we say both Truck have mass M kg. Then the initial momentum is (6M+0M)kgms^-1= 6M kgms^-1
After the collision we have the mass of the whole 'body' = 2M kg
and we'll say the 'body' is travelling at Vms^-1. Therefore momentum after collision = 2MV kgms^-1
As momentum is conserved
6M = 2MV --> 3=V
Now, make sure you understand the working by trying some other examples.