If the electric field is given by $(5\hat{\mathbf{i}}+4\hat{\mathbf{j}}+9\hat{\mathbf{k}})$,  then the electric flux through a surface of area 20 unit lying in the yz- plane will be 

Electric flux is equal to the scalar product of an area vector  and electric field E. As the surface is lying in yz-plane,

$\therefore \varphi =\mathbf{E}·\mathit{d}\mathbf{A}=(5\hat{\mathbf{i}}+4\hat{\mathbf{j}}+9\hat{\mathbf{k}}).\left(20\hat{\mathbf{i}}\right)=100\text{ unit }$