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.

On the
axis,
with
the potential
is

and
so we may find the point where

or

Solving
the quadratic, we
get:

Since
we started out assuming
we must choose the plus sign, and
then