A1200-kg cannon suddenly fires a cannonball at what is the recoil speed of the cannon? assume that frictional forces are negligible and the cannon is fired horizontally.

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.