Two particles of masses m1 and m2 are located at r1 and r2 relative to some origin

Question:

Two particles of masses m1 and m2 are located at r1 and r2 relative to some origin O as in Figure. They exert equal and opposite forces on each other. Calculate the resultant torque exerted by these internal forces about the origin O and show that it is zero if the forces F1 and F2 lie along the line joining theparticles.