Walker, Halliday & Resnick — Electric Fields: Problem 26

26 Figure 22-36 shows two concentric rings, of radii $R$ and $R' = 4.00R$, that lie on the same plane. Point $P$ lies on the central $z$ axis, at distance $D = 2.00R$ from the center of the rings. The smaller ring has uniformly distributed charge $+Q$. In terms of $Q$, what is the uniformly distributed charge on the larger ring if the net electric field at point $P$ is zero?