A uniform electric field $E = 2 \times {10}^3 N C^{-1}$ is acting along the positive $x-axis$. The flux of this field through a square of $10 cm$ on a side whose plane is parallel to the $y-z plane$ is 

Here, $E = 2 \times {10}^3 N C^{-1}$ is along $+ x-axis$

 Surface area, $S = {(10 cm)}^2 = {10}^2 \times {10}^{-4} m^2 = {10}^{-2} m^2$

 When plane is parallel to $y-z plane,\theta =0^0$ with $+ x-axis$

 So $\varphi =E S \cos \theta =2\times {10}^3\times {10}^{-2} \cos 0^{°}=20 N C^{-1} m^2$