Expression for an electric field is given by E → = 4000   x 2   i ^   V   m - 1 .…

Electric flux is expressed as $\varphi =\overrightarrow{E}·\overrightarrow{A}=EA\cos \theta$, here, $A$ is area of the surface and $\theta$ is the angle between the electric field lines and the normal to $A$.

The flux will be only from DEFG surface as on the surface OABC field is zero and for rest of the surface, area vector is perpendicular to field. 

So, $\varphi =EA$

$=4000\times (0.2{)}^2\times (0.2{)}^2$

$=4000\times 16\times {10}^{-4} \mathrm{V} \mathrm{m}$

$=640 \mathrm{V} \mathrm{cm}$