With the engines off, a spaceship is coasting at avelocity of +260m/s through outer space. The ship carries rockets that are mounted in firing tubes, the back ends of which are closed. It fires a rocket straight ahead at an enemy vessel. The mass of the rocket is 1100kg, and the mass of the spaceship (not including the rocket) is 5000000kg. The firing of the rocket brings the spaceship to a halt. What is the velocity of the rocket?

Momentum is conserved.
MassRocket*changevelocityrocket=massShip(changevelocity sship)

Now the change in rocket velocity is (Vr-260m/s). The change in ship velocity is -260m/s

solve for velocity of the rocket.