The question also says that the cannonball has a mass of 100.0kg and is fired at a speed of 35m/s.
Consider the conservation of momentum:
P₁ = P₂
where:
1 = cannonball
2 = cannon
Therefore:
m₁ · v₁ = m₂ · v₂
Solve for v₂:
v₂ = (m₁ · v₁) / m₂
= 100 · 35 / 1200
= 2.9 m/s
The cannon will have a recoil speed of 2.9 m/s.