By Gauss law in electrostatics, electric flux through any closed surface is
$\frac{q}{\varepsilon_{0}}$.
If the charge is located at the center of the cube it is placed symmetrically with respect to each other.
$\therefore $ Electric flux through each face is
$\frac{1}{6} \frac{q}{\varepsilon_{0}}$
$=\frac{1}{6} \frac{4 \pi q}{\left(4 \pi \varepsilon_{0}\right)}$