To find the missing velocity after the collision, we apply the law of conservation of momentum, which states that the total momentum before the collision must equal the total momentum after the collision. We first calculate the momentum of each object before the collision. For the first object with a mass of 300 kg moving at 10 m/s, the momentum is 300 kg * 10 m/s = 3000 kg*m/s. For the second object with a mass of 100 kg moving at -6 m/s, the momentum is 100 kg * -6 m/s = -600 kg*m/s. The total momentum before the collision is 3000 kg*m/s + (-600 kg*m/s) = 2400 kg*m/s. After the collision, the two objects stick together, so their combined mass is 300 kg + 100 kg = 400 kg. Setting the total momentum before the collision equal to the total momentum after the collision gives us the equation: 2400 kg*m/s = 400 kg * velocity after collision. Solving for the velocity after collision, we find that it equals 2400 kg*m/s / 400 kg = 6 m/s. However, the answer provided in the choices does not match this calculation, indicating a potential error in the choices or the question setup.