The charges + 4q,−q and +4q are kept on a straight line at position (0,0,0),(a,0,0) and (2a,0,0) respectively. Considering that they are free to move along the x−axis only
Attraction between, (+4qand−q), F1=k(+4q)(−q)a2=4kq2a2
Repulsion between, (+4qand+4q),F2=k(+4q)(+4q)(2a)2=4kq2a2
Net repulsion and attraction force is zero,
Hence, System is in equilibrium