What is the electric field at q due to 4 charges in a square configuration?

[Scarth]

http://puu.sh/7MuKv.jpg
I'm not sure what equation to apply in this situation. I'm lead to believe
K_e((|q₁||q₂|) / a)
Is what I should be applying.

The Attempt at a Solution

No idea thus far.

 

[Simon Bridge]

Welcome to PF;

I'm not sure what equation to apply in this situation. I'm lead to believe
Ke((|q1||q2|) / a)
Is what I should be applying.

What leads you to believe that is the right equation?

Do you know the definition of the electric field?
If you do not, look it up. (online or in your course notes or both)

The electric field at q is the vector sum of the electric fields due to the other charges.
Notice that the distance between Bq and q is not a.