> A point charge is placed at each corner of a square with side length
> $a$. The charges all have the same magnitude $q$. Two of the charges
> are positive and two are negative, as shown in the following figure.
> 
> What is the magnitude of the net electric field at the center of the
> square due to the four charges in terms of $q$ and $a$?

I'm trying to understand this question by using the solution my instructor gave me, but I'm confused.

I found $r=\frac{a}{\sqrt{2}}$, which makes sense. However then it says:

> The magnitude of the electric field due to each charge is the same and equal to 

> $$E_q = \frac{kq}{r^2} = \frac{2kq}{a^2}$$

How did they come up with this? I only know the equations $F = \frac{k\lvert q_1q_2\rvert}{r^2}$ and $E = \frac{F_0}{q_0}$ and I can't seem to work out how they got to this point.