the three blocks (from left to right in the figure) have masses of 3.12 kg, 0.748 kg and 7.49 kg. the leftmost block has a velocity of 3.32 m/s toward the right, the center block is stationary, and the rightmost block as a velocity of 2.43 m/s toward the left. assuming movement toward the right is positive, what is the velocity of the center-of-mass of this system as pictured (prior to collision)? give your answer with the appropriate sign.