Net Electrostatic Force on Four Charges at the Corners of a Square

[seb26]

Homework Statement

There are four charges, each with a magnitude of 3.04C. Two are positive and two are negative. The charges are fixed to the corners of a 0.124-m square, one to a corner, in such a way that the net force on any charge is directed toward the center of the square. Find the magnitude of the net electrostatic force experienced by any charge.

Homework Equations

1) Fe=(k*q1*q2)/r²
2) E=Q/(e*A) Q(total charges); A(area); e(permittivity of free space)

The Attempt at a Solution

I put the positive charges in diagonally opposite corners, same for the negative charges.
My first attempt was to find out the y and x-axis of one positive charge by using the first equation and calculating the net force.
Then I used again the first equation for the net force between the diagonally opposite positive charges. I subtracted my second result from the first.
I thought this would be the net force experienced by every any charge. But no

Then I thought that by looking at the square as being a Gaussian surface I could use the equation but wouldn't the charges cancel out?

 

[Spinnor]

Draw a picture. Pick any charge and draw the three forces acting on it. The magnitude of these forces should be easy and the directions obvious? The vector sum of these forces points towards the middle. A little bit of work and you should have the magnitude.