If Newton's 3rdLaw of Motion is written in the form of F21 = F12; what is the mistake?

It should be here.
F21 = -F12
‚ÄčThis shows that force on a body A by B is equal an opposite to the force on body by A. Hence, the forces of action and reaction are equal and opposite, which proves Newton's 3rd law of motion.
 

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