1. The problem statement, all variables and given/known data 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. 2. Relevant equations E = kq/r^2 3. 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?