Answer
$-8.85\;\rm nC$
Work Step by Step
We know that the net electric flux through the closed surface is given by
$$\Phi=\dfrac{Q_{in}}{\epsilon_0}$$
Hence, the enclosed charge inside of our octahedron is given by
$$Q_{in}=\Phi\epsilon_0$$
Plugging the known;
$$Q_{in}=(-1000)(8.85\times 10^{-12})$$
$$Q_{in}=\color{red}{\bf -8.85}\;\rm nC$$
