DS is separately zero for each face
of the cube except the two shaded ones Now the magnitude of
the electric field at the left face is
EL = ax1/2 = aa1/2
(x = a at the left face) The magnitude of electric field at the right face is
ER = a x1/2 = a (2a)1/2
(x = 2a at the right face) The corresponding fluxes are
fL= EL