A system consists of two particles. At = 0 one particle is at the origin; the other, which has a mass of 0.50 , is on the -axis at = 6.0. At = 0 the center of mass of the system is on the -axis at = 2.4. The velocity of the center of mass is given by ( 0.75 ) . a.) Find the total mass of the system b.) Find the acceleration of the center of mass at any time t (express answer using two sig figs) c.) Find the net external force acting on the system at t=3.0 s