Cannon Question: Loose Cannon Velocity

[JessGold]

Homework Statement

A cannon of mass 5.80 x 10^3 kg is rigidly bolted to the Earth so it can recoil only by a negligible amount. The cannon fires an 85.0-kg shell horizontally with an initial velocity of +551 m/s. Suppose the cannon is then unbolted from the Earth and no external force hinders its recoil. What would be the velocity of a shell fired by this loose cannon? (Hint: In both cases assume that the burning gunpowder imparts the same kinetic energy to the system.)

Homework Equations

The Attempt at a Solution

 

[CompuChip]

Two conservation laws apply here. One of them is already implicitly mentioned in the hint. The other one has to do with the masses and velocities.