In otherwise empty space is a system of 4 particles, each of the same mass. The accelerations of the particles are as follows: a1 = ˆi a a2 = ˆj a 2 a3 = −( ˆi +ˆj) (2 a) a4 = (ˆi −ˆj) (3 a), where a > 0 is a constant. What is the acceleration of the center of mass of the system of 4 particles?