Fig. 3-2a illustrates a red train that is moving forward at a speed given by v / c = β = s / a. From the primed frame of the train, a passenger shoots a bullet with a speed given by u ′ / c = β ′ = n / m, where the distance is measured along a line parallel to the red x ′ axis rather than parallel to the black x axis. What is the composite velocity u of the bullet relative to the platform, as represented by the blue arrow? Referring to Fig. 3-2b: