A cubical shell with edges of length a is positioned so that two adjacent sides of one face are coincident with the +x and +y axes of a rectangular coordinate system and the corner formed by these two sides is at the origin. An electric field of magnitude E=bx2 directed along the +x axis exists in this region. How much charge is contained in the volume of the shell?
Express your answer in terms of the variables a, b, and the permittivity constant ϵ0.