A rocket experienced a launch anomaly and exploded into four parts. The first part has a mass of 4000 kg and a velocity of 7.5 km/s ĵ. The second part has a mass of 3500 kg and a velocity of 1.1 km/s î +4.5 km/s ĵ. The third part has a mass of 5000 kg and a velocity of -1.2  km/s î +3.5 km/s ĵ. The velocity of the fourth part of mass 500 kg is -0.5 km/s ĵ. Calculate the velocity of the rocket prior to the explosion.