Two cars are in a collision at the intersection of Boyd and Elm, tying up after-class traffic. The first car, mass 927 kg, was heading north on Elm at 11.55 m/s and the second car, mass 1,204 kg, was heading east on Boyd at 15.01 m/s. Assuming that after the collision the two cars are locked together, moving with the same final velocity, what is the magnitude of their velocity (speed) immediately after the collision?