1. The problem statement, all variables and given/known data http://www.freeimagehosting.net/uploads/3e00ce8695.gif 2. Relevant equations Electric field equation derived from Coloumb's Law (given on the screenshot above). 3. The attempt at a solution Solution is given in the form of answer sheet: http://www.freeimagehosting.net/uploads/0d23171fd2.gif My problem is - I can't understand how the result in point b) is derived. My best guess was deriving an equation for Ey and equate it to 0 (taking into account the new values for a, b and Q). From there a relation between b2 and b can be derived. But I really don't understand why the result is fourth root of 2. My guess was square rooth of 2? It's unclear to me but it seems that if Ey has to be negated, than Q2 has an opposite charge compared to the other ones? Where is this "point of origin"? I'm really confused :(.