(a) Since the body is moving with no acceleration, the sum of the forces is zero =F_{1} + F_{2} + F_{3} = 0. Let F_{1}, F_{2}, F_{3} be the three forces passing through a point. Let F_{1} and F_{2} be in the plane A (one can always draw a plane having two intersecting lines such that the two lines lie on the plane). Then F_{1} + F_{2} must be in the plane A. Since F = - (F_{1} + F_{2}), F_{3} F is also in the plane A.

(b) Consider the torque of the forces about P. Since all the forces pass through P, the torque is zero. Now consider torque about another point 0. Then torque about 0 is

Torque = OP x (F_{1} + F_{2} + F_{3})

Since F_{1} + F_{2} + F_{3}