A slab of insulating material has thickness 2d and is oriented so that its faces are parallel to the yz-plane and given by the planes x = d and x = -d. The y- and z-dimensions of the slab are very large compared to d and may be treated as eesentially infinite. The slab has a uniform positive charge density p. Now let the charge density of the slab be given by p(x) = p0(x/d)^2, where p0 is a positive constant.