Conservation of momentum requires that the sum of momenta after is equal to that before. Â Since initially nothing is moving, the sum after the shot will also add to zero.Â
mâvâ = -mâvâ
Solve for the cannon's velocity vâ
vâ = -mâvâ/mâ = -2.10m/s
The negative sign means it's moving 2.10m/s south.