Two point charges 2 q and q are placed at vertex A and centre of...

Using Gauss's law:
$\phi = \frac{Q_{in}}{\epsilon_0}$

Charge $2q$ is at vertex A of the cube. A vertex is shared by 8 cubes, so the flux contribution from $2q$ through this cube is $\frac{2q}{8\epsilon_0} = \frac{q}{4\epsilon_0}$.

Charge $q$ is at the centre of face CDEF. A face center is shared by 2 cubes, so the flux contribution from $q$ through this cube is $\frac{q}{2\epsilon_0}$.

Total flux:
$\phi = \frac{q/4 + q/2}{\epsilon_0} = \frac{3q}{4\epsilon_0}$