Answer
$8.85\;\rm nC$
Work Step by Step
We know that the net electric flux through a closed surface is given by
$$\Phi=\dfrac{Q_{in}}{\epsilon_0}$$
We have a box of 6 faces and the flux will be divided on these 6 faces as the author told us 300 N.m$^2$/C for 2 faces and 100 N.m$^2$/C for the other 4 faces.
Thus,
$$\Phi_{\rm box}=2(300 )+4(100)=\dfrac{Q_{in}}{ \epsilon_0}$$
Thus the charge inside the box is given by
$$Q_{in}= [600+400]\epsilon_0$$
Plug the known;
$$Q_{in}= 1000 (8.85\times 10^{-12})$$
$$Q_{in}=\color{red}{\bf 8.85}\;\rm nC$$
