The reaction force will have magnitude of 300 N and its direction will be to the left.
According to the third Newton's Law, every force (action) from object 1 to object 2 causes another force (reaction), from object 2 to object 1. These forces have to be equal in magnitude and opposite in direction:
`vecF_(12) = -vecF_(21)` .
This equation implies that the magnitudes are the same: `F_(12) = F_(21)` and directions are opposite: if one force is to the right, then the other one is to the left.
The action and reaction force are sometimes also called "third Newton's law pair".
It is important to note that the forces in a third Newton's Law pair are always applied to the different objects, never the same object. In the example above, the force `vecF_(12)` is acting on object 1, and the force `vecF_(21)` is acting on object 2.