Chapter 13, Problem 13.9E

(a)

To determine

The position of third mass so that the net force on system is zero.

(b)

To determine

For the equilibrium condition of third mass $\left(M\right)$ the system is stable or unstable when points are at connecting line of particle first $\left(m\right)$ and second $\left(3m\right)$ and when points are at connecting line via third mass $\left(M\right)$ and perpendicular to the connecting line.