Conservation of momentum requires that the sum of momenta after is equal to that before. Since initially nothing is moving, the sum after the shot will also add to zero.
m₁v₁ = -m₂v₂
Solve for the cannon's velocity v₁
v₁ = -m₂v₂/m₁ = -2.10m/s
The negative sign means it's moving 2.10m/s south.
