Two concentric rings of radii r and r' 

The figure shows two concentric rings, of radii R and R’ = 3.00R, 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 P is zero?