Final answer:
Equilibrium along the line connecting M and 3M is stable.
Step-by-step explanation:
The equilibrium of an object can be categorized as stable or unstable. If an object at equilibrium is displaced from its position, a stable equilibrium means it will return to its original position, while an unstable equilibrium means it will move further away from its original position.
Along the line connecting point M and point 3M, the equilibrium can be described as stable. If a point on this line is displaced, it will experience a force pointing back toward the equilibrium point, which makes it stable.