suppose the equilibrium position of a particle is at P.
then it is said to be stable, when.... if we displace it slightly in a direction.(in this case we can only displace it along y axis) .and..it has a tendency to come back to its equilibrium position . like. for eg. in case of two identical charges separated by a distance d apart. if we place a point charge at the mid point of line joining them.. it will be at equilibrium.. and if we displace it slightly along the line. it will come back to its equilibrium position.. this is called stable.
but in case of neutral equilibrium.. if you displace the particle in a direction.and move it to, say, point R.. . it will then also remain in equilibrium at R. for eg. consider a particle which is in equilibrium due to gravitational and electrical forces.in a constant gravitational and electric field.. . then even if u displace it anywhere.. u will find it in equilibrium..(as the denominator containing "r²" term in the force equation will cancel out)
got the difference between the two terms? @Ananya_Singh
now thinknabout the line charge question.. u will get the amswer.. if not.. then feel free to ask again