Answer
$q=-6.64\times 10^{-9}C$
Work Step by Step
According to Gauss's law, the flux through any closed surface is given as: $\phi=\frac{q}{\epsilon_{\circ}}$
This can be rearranged as:
$q=\epsilon_{\circ}\phi$
We plug in the known values to obtain:
$q=8.85\times 10^{-12}(-750)=-6.64\times 10^{-9}C$