Answer
$188\;\rm N\cdot m^2/C$
Work Step by Step
We know that the net electric flux through a closed surface is given by
$$\Phi=\dfrac{Q_{in}}{\epsilon_0}$$
And we have a box of 6 identical surfaces that contains a charge of 10 nC at its center. Thus the flux will be divided on these 6 surfaces equally.
Thus,
$$\Phi_{\rm surface}=\dfrac{Q_{in}}{6\epsilon_0}$$
Plug the known;
$$\Phi_{\rm surface}=\dfrac{10\times 10^{-9}}{6(8.85\times 10^{-12})}$$
$$\Phi_{\rm surface}=\color{red}{\bf 188}\;\rm N\cdot m^2/C$$
