What is the total force exerted by two charges on a point charge at the origin?

[bphysics]

Homework Statement

Two point charges are placed on the x-axis as follows: one positive charge, q₁, is located to the right of the origin at x= x₁, and a second positive charge, q₂, is located to the left of the origin at x= x₂.

What is the total force (magnitude and direction) exerted by these two charges on a negative point charge, q₃, that is placed at the origin? Use \epsilon_{0} for the permittivity of free space. Take positive forces to be along the positive x-axis. Do not use unit vectors.

Homework Equations

Coulomb's Law: F = \frac{1}{4\pi\epsilon_{0}}\frac{\left|q_{1}q_{2}\right|}{r^{2}}

The Attempt at a Solution

http://img5.imageshack.us/img5/3753/equationu.jpg

MasteringPhysics keeps giving me a "Check your signs error". Yet, as far as I can tell, I should be subtracting the force which is going left / the negative direction (ie, q₂) from the force going to the right (ie, q₁).

Any hints?

 

[bphysics]

My apologizes if the "attempt at a solution" section appeared blank for anyone -- I have changed the link so that users who are not logged in can now see the link.

 

[thebigstar25]

u can find the electric field at the origin from the charge q1 and q2 and then u can find the force using F= q3 * E .. try this one, I hope it will work