Two electric charges $q$ and $−2q$ are placed at $(0,0)$ and $(6,0)$ on the $x-y$ plane. The equation of the zero equipotential curve in the $x-y$ plane is

1. $x=-2$
2. $y=2$
3. $x^{2}+y^{2}=2$
4. $(x+2)^{2}+y^{2}=16$

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