A charge Q is placed at a distance a/2 above the centre of the square surface of edge a as shown in the figure. 

The electric flux through the square surface is $\frac{\mathrm{Q}}{{\mathrm{K\epsilon }}_{\mathrm{o}}}\mathrm{K}=?$

When cube is of side a an point charge Q is at the center pf the cube then the total electric flux due to this charge will pass evenly through the six faces of the cube. so, the electric flux through one face will be equal to 1/6 of the total electric flux due to this charge.

Flux through 6faces $=\frac{\mathrm{Q}}{{\mathrm{ϵ}}_0} \therefore$Flux through 1 face $=\frac{\mathrm{Q}}{6{\mathrm{ϵ}}_0}$