Force equals mass times acceleration.
The principle of inertia.
Law of action and reaction.
This is the fundamental formula of dynamics, and it comes from assuming a body at rest on a horizontal surface, which is subject to a force parallel to that surface, being able to dispense with friction: we will see that the body starts moving at a constant acceleration.
It indicates that when a body is at rest, or describes a movement with the characteristics of MRU, the forces applied to it have a null result.
Whenever one body exerts a force on another, this second exerts an opposite of equal intensity and direction but opposite sense on the first.