An object of mass 1.25 kg on a flat surface has two forces acting on it, F1 and F2 in the horizantal direction. F1 acts in the positive direction, F2 acts in the negative direction. The magnitude of F1 is 26.75 N. The object moves strictly along the horizontal x-axis, which we choose as positive to the right. Find the magnitude of F2 if the object has a horizontal acceleration of 9.3 m/s2.

F1+F2 = M*a.

26.75 + F2 = 1.25 * 9.3 = 11.625.
F2 = 11.625 - 26.75 = -15.125 N. = 15.125 Due West.