Only the top and bottom ends of the cube have nonzero flux, and both are positive, for a total flux of \(\Phi_E = 2EL^2\text{,}\) where \(E\) is the magnitude of the electric field a distance \(L/2\) from the sheet. The total charge enclosed by the box is the charge density times the cross-sectional area \(Q_{enc} = \sigma_o L^2\text{.}\)