Answer
$1.64\times10^{-8}C$
Work Step by Step
Use Gauss’s law to find the enclosed charge. The size of the box does not matter.
$$\Phi_E=\frac{Q_{enc}}{\epsilon_o}$$
$$Q_{enc}=\Phi_E\epsilon_o$$
$$=(1.85\times10^3 N \cdot m^2/C)(8.85\times10^{-12}C^2/(N \cdot m^2)) $$
$$=1.64\times10^{-8}C$$
