A 1.0 103 kg Toyota collides into the rear end of a 2.2 103 kg Cadillac stopped at a red light. The bumpers lock, the brakes are locked, and the two cars skid forward 3.6 m before stopping. The police officer, knowing that the coefficient of kinetic friction between tires and road is 0.44, calculates the speed of the Toyota at impact. What was that speed?