The initial momentum is **p**=m_{1}**v**_{1}+m_{2}**v**_{2}**, **where by boldfaced letters I denote vector quantities. Since the second
blob is at rest, **v**_{2}=0. Then

**p**=m_{1}**v**_{1}; |p|=m_{1}|**v**_{1}|=2*3=6 kg*m/s;

In the closed system the total momentum is conserved. Therefore, the final momentum is also 6 kg*m/s, directed the same way as the initial momentum. However, since two blobs are now stuck together, the total mass is now M=m_{1}+m_{2}=2+2=4
kg. Therefore,

p=Mv=6=4v; From this v=6/4=1.5 m/s, directed the same way as the initial velocity of the first blob.