Two horizontal forces, and , are acting on a box, but only is shown in the drawing. ...

Two horizontal forces, and , are acting on a box, but only is shown in the drawing. 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 = +9.9 N and the mass of the box is 3.8 kg. Find the magnitude and direction of when the acceleration of the box is (a) +6.2 m/s2, (b) -6.2 m/s2, and (c) 0 m /s2