The potential is negative in the region surrounding the central, negative charge, and is positive surrounding each of the positive charges. At a great distance, the potential is positive and approaches the value for a single positive point charge Thus an equipotential surface on which the potential is zero surrounds the negative charge, as shown in the diagram.
and so we may find the point where
Solving the quadratic, we get:
Since we started out assuming we must choose the plus sign, and then