The key is in "they lock together". This tells you this is a complete inelastic collision and therefore you can use the formula:
m1*v1+m2*v2=v(m1+m2)
Plug in numbers:
2000*3+2000*2=v(2000+2000)
so v=2 m/s, obviously in the same direction of motion in which they were both moving.
