Three charges are arranged in a linear array. The charge $-2$q is placed at the origin and two charges, each of $+$q$,$ are placed at $(0,0,$ l$)$ and $(0,0,$l$),$ respectively. $($a$)$ Find a relatively simple expression for the potential $$phi$($X$)$ that is valid for distances $|$X$|$ I. $($b$)$ Draw a graph of the equipotential surfaces in the x$,$ y plane.

Please be clear in your developments.