The potential energy question-?
The potential energy of a particle in a certain field is given by
U=a/r^2 - b/r
where a and b are +constants and r is the distance from the centre of the field.
The distance of particle in the stable equilibrium position is-
1) a/b
2) -a/b
3) 2a/b
4) -2a/b

You want U = 0 for equilibrium, which of your answers satisfies that?