Electrostatic problem (point charges)

[martinskibg]

Homework Statement

http://www.freeimagehosting.net/uploads/3e00ce8695.gif

Homework Equations

Electric field equation derived from Coloumb's Law (given on the screenshot above).

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 :(.

 

[gabbagabbahey]

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.

That seems like a good plan to me. Just remember, that at the origin $x=y=0$.

Also, you might want to double check the solution to part (a) that you posted, because I see two missing negative signs after just a quick glance at it, and there may be more errors.