To calculate the magnitude of the initial momentum of the car in this situation you have to multiple the mass of the blue car times the speed. The mass, 468kg, times the speed, 21 m/s, equals 9828. This means the magnitude of the initial momentum of the blue car in the situation is 9828kg per m/s. In this case, and in simple terminology, the magnitude measures how big something is. So the magnitude of the momentum is just the measurement of how big the initial moment of the car is.
