Walker, Halliday & Resnick — Electric Fields: Problem 9

9 Figure 22-25 shows two parallel nonconducting rings with their central axes along a common line. Ring 1 has uniform charge $q_1$ and radius $R$; ring 2 has uniform charge $q_2$ and the same radius $R$. The rings are separated by distance $d = 4.00R$. The net electric field at point $P$ on the common line, at distance $R$ from ring 1, is zero. What is the ratio $q_1/q_2$?