Two horizontal forces, f1 and ,f2 are acting on a box, but only is shown in the drawing. F2 can point either to the right or to the left. The box moves only along the x axis. There is no friction between the box and the surface. Suppose that F2 = +6.8 N and the mass of the box is 3.9 kg. Find the magnitude and direction of F2 when the acceleration of the box is (a) +4.2 m/s2, (b) -4.2 m/s2, and (c) 0 m /s2