A square with charges and an electric field

[Melawrghk]

Homework Statement

This is a problem from my physics midterm. I have no idea whether I got ANYTHING at all right in it, although I certainly hope so.

Homework Equations

E = kq/r^2

The Attempt at a Solution

Okey dokey. I was running out of time (majorly) so I figured I'd only look at charge 1's electric field at the origin. The main theory (or something like that) behind it was that charges 2 and 4 lay in a line, so their electric fields would coincide, so charge 4 could be used to cancel out 2's effects.
I then decided that charge 3 basically has to counterbalance charge 1's electric field. So I found 1's electric field at the origin:
E₁ = (k*10e-9)/7.25 = -12.4V/C
I then split it into components, x & y, because that is what charge 3 would have to counteract.
E₁ = -4.61i - 11.51j

Long story short, I arrived at the conclusion that q(3) should be -10e-9 C. But I'm not sure if that's at all right. Thoughts? Feedback?

 

[Delphi51]

So you can choose Q4 as well as Q3?
I think your choice for Q3 is right on. The combined E's due to Q1, Q2 and Q3 will be in the direction of Q4 so they can be canceled by choosing just the right Q4.

 

[Melawrghk]

Yep, both q3 & q4 were unspecified. The question only asked about q3 though.
Thanks for the reply! :) That makes me feel a little better about my overall predicament.