In a region of space the electric field is given

$$\begin{gathered} \overrightarrow{\mathrm{E}}=8\widehat{\mathrm{i}}+4\widehat{\mathrm{j}}+3\widehat{\mathrm{k}}, \\ \overrightarrow{\mathrm{A}}=100\widehat{\mathrm{k}} \end{gathered}$$

The electric flux for the x-y plane,

$\begin{array}{l}\varphi =\overrightarrow{E}\cdot \overrightarrow{A}\\ \varphi =(8\widehat{ı}+4\widehat{ȷ}+3\widehat{k})\cdot \left(100\widehat{k}\right)\end{array}$

$\mathrm{\varphi }=300$ units

So, the electric flux through a surface of area 100 units in x-y plane is 300 units.