Electric dipole charges/Electric Field

Homework Statement

Two point charges likes those in the figure below are called an electric dipole. Show that the electric field at a distant point along the x-axis is given by $$E_{x}=\frac{4k_{e}qa}{x^3}$$
Figure: http://img300.imageshack.us/my.php?image=58ag9.png

Homework Equations

Electric field equation: $$E=\frac{k_{e}q}{r^2}$$
Anyothers?

The Attempt at a Solution

I'm unsure of how to apply the electric field equation to this problem (if it is even going to be used). I'm unfamiliar with electric dipoles and certaintly haven't been dealing with the electric fields of them. Could someone give me a hint as to where I should start on this problem? I appreciate it, thanks!

P.s. I think that this may be in relation with this problem: http://www.sciforums.com/showthread.php?t=62789 but I'm unsure of what they mean and why the distance on the bottom of the fraction is cubed.