Solving for Spring Constant in Hooke's Law with Charged Spheres

[psilovethomas]

Homework Statement

A tiny sphere with a charge of q = +8.8 µC is attached to a spring. Two other tiny charged spheres, each with a charge of −4.0-µC, are placed in the positions shown in the figure, in which b = 4.1 cm. The spring stretches 5.0 cm from its previous equilibrium position toward the two spheres. Calculate the spring constant.
Here is the figure:
http://www.webassign.net/grr/p16-16alt.gif

Homework Equations

The Attempt at a Solution

µΔx=k(Qq)/r

 

[pgardn]

Homework Statement

A tiny sphere with a charge of q = +8.8 µC is attached to a spring. Two other tiny charged spheres, each with a charge of −4.0-µC, are placed in the positions shown in the figure, in which b = 4.1 cm. The spring stretches 5.0 cm from its previous equilibrium position toward the two spheres. Calculate the spring constant.
Here is the figure:
http://www.webassign.net/grr/p16-16alt.gif

Homework Equations

The Attempt at a Solution

µΔx=k(Qq)/r

they are saying tiny charges so I guess we can assume gravity does not play a role.

What force is pulling the charge up and what force is pulling the charge down when it is in equilibrium after stretching?