Four positive point charges $+q$ are kept at the four corners of a square of side $l$. The net electric…

The electric field of $E_C \& E_D$ are equal and opposite so they cancel out each other,

now net electric field at the midpoint of one side is,

$2E_A\cos \theta$ because $E_A=E_B$, so,

$$\begin{gathered} E_{net}=2\times k\times \frac{q}{\sqrt{l^2+{\left({\displaystyle \frac{l}{2}}\right)}^2}}\cos \theta \\ {E}_{net}=2\times k\times \frac{q}{{\left(\sqrt{l^2+{\left({\displaystyle \frac{l}{2}}\right)}^2}\right)}^2}\times \frac{l}{\sqrt{l^2+{\left(\frac{l}{2}\right)}^2}} \\ {E}_{net}=\frac{16 kq}{5\sqrt{5}{l}^2} \end{gathered}$$