As shown in the figure below, blocks A and B move on a frictionless horizontal surface and make a completely inelastic collision. Their masses are mA = 1.0 kg and mB = 3.0 kg. Before collision, their x-velocities are vA1x = 5.0 m/s and vB1x = −1.0 m/s. After the completely inelastic collision, both blocks stick together and move with velocity v2x. What is v2x?