[tex]\left\{\begin{aligned}&P_{\square{}}= 12\text{ mm}\\&P_{\square{}} =4l\end{aligned}\,\right| \Rightarrow 4l = 12 \text{ mm}\Rightarrow l = 3\text{ mm}[/tex]
[tex]\left\{\begin{aligned}&A_{\square{}}= l^2\\&l = 3\text{ mm}\end{aligned}\,\right| \Rightarrow A_{\square{}} =(3\text{ mm})^2 \Rightarrow \boxed{A_{\square{}} =9\text{ }\mathrm{mm}^2}[/tex]
