Question

what is the third law of Newton

Answer 1

For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.

Answer 2

action and reaction are of equal magnitude directed in opposite direction.
i.e. if body apply Fab force on another body then second body apply same magnitude of force Fba but direction is opposite .
hence
Fab=-Fba