Solution :
Total surface flux linked with a closed with a closed surface is given by $\phi = \large\frac{q}{\in_0}$
Let $q$ be the charge placed at the center of the cube of which is a guassian surface.
given $ q= 2 \mu C= 2 \times 10^{-6}$
length of the edge = 9 cm
net electric flux through this surface is
$\phi =\large\frac{q}{\in_0} =\frac{2 \times 10^{-6}}{8.854 \times 1^{-12}}$
$\qquad= 2.26 \times 10^5 Nm^2/C$
Hence the net electric flux through the surface is $2.26 \times 10^5 Nm^2/C$