A point charge of +12 nC sits at the centre of a cube whose edges are 0.30 m long…

Step 1. The whole cube. $\Phi = \dfrac{q}{\varepsilon_0} = \dfrac{12 \times 10^{-9}\ \text{C}}{8.85 \times 10^{-12}\ \text{C}^2/(\text{N m}^2)} = 1356\ \text{N m}^2/\text{C}$.

Step 2. One face. The charge is at the centre, so all six faces are alike. $\dfrac{1356}{6} = 226\ \text{N m}^2/\text{C}$.

Step 3. Both charges. The net charge inside is $12 - 4.0 = 8.0$ nC. The flux through the cube is $\dfrac{8.0 \times 10^{-9}}{8.85 \times 10^{-12}} = 904\ \text{N m}^2/\text{C}$. One face gets $\dfrac{904}{6} = 151\ \text{N m}^2/\text{C}$.