Walker, Halliday & Resnick — Electric Potential: Problem 8

8 Two particles, of charges $q_1$ and $q_2$, are separated by distance $d$ in Fig. 24-26. The net electric field due to the particles is zero at $x = d/4$. (a) With $V = 0$ at infinity, locate (in terms of $d$) any point on the $x$ axis (other than at infinity) at which the electric potential due to the two particles is zero. (b) If, instead, the net electric potential is 0 at $x = d/4$, find (in terms of $d$) the coordinate of any point (other than at infinity) at which the net electric field is zero.