The reason it comes out as a quadratic equation because there are two places that satisfy the problem
(You can place it at a certain distance to the left of the charges, or a certain distance to the right of the charges.)
The reason your quadratic was imaginary is because you set up the initial equation wrong.
You want [itex]F_1+F_2=0[/itex] and therefore [itex]F_1=-F_2[/itex] (Whereas you wrote "[itex]F_1=F_2[/itex]" without the negative sign)
P.S.
In the future, it would be very helpful if you typed out your thoughts in the thread. The reason is that it can sometimes be difficult to read off of a picture. (And I almost didn't even notice that there were words in your picture!) Thank you :)