Electric Charge and Electric Field Problem

[lawk]

1. The problem statement, all variables and given/known data

Two small spheres, each of mass 8.00 g , are suspended by light strings 15.0 cm in length (see figure). A uniform electric field is applied in the x direction. The spheres have charges equal to -6*10^-8 and 6*10^-8 Determine the electric field that enables the spheres to be in equilibrium at an angle of 11 degree.

http://i35.tinypic.com/5ewwzn.jpg

2. Relevant equations

ke = 8.99*10^9
gravity= 9.8 m/s^2

ke(q1*q2)/r^2 ?

3. The attempt at a solution

I don't know how to approach the problem.
I don't know how gravity is affecting the problem.
Can someone guide me through the problem???
What should I do in order to get the answer?

[Defennder]

If you examine the diagram, you'll see that there are 3 forces acting on the system. Two of them, electrostatic attraction between the positive and negative charges and gravity cause the spheres to move closer together, and downwards respectively. The 3rd force, that due to the applied E-field, repels the positive charge and attracts the negative one. Those 3 forces in conjunction create a static equilibrium. Your job is to find the magnitude of the E-field applied.

Note that you must take into account the E-field due to either the positive or negative charge (unless it's small enough to be negligible).