[tex]l = \dfrac{3}{5}\cdot L $ m\\ \\ \left\{ \begin{array}{c} A_{\boxed{~}} = l\cdot L $ m^2}\quad $ $ $ $\\ A_{\boxed{~}} = 1215 $ m^2 \end{array} \right| \Rightarrow \left\{ \begin{array}{c} l\cdot L = 1215 $ m^2}\\l=\dfrac{3}{5}\cdot L$ m\quad $ $ $ $ \end{array} \right| \Rightarrow \dfrac{3}{5}\cdot L\cdot L = 1215$ m^2 \Rightarrow \\ \\ \\ \Rightarrow \dfrac{3}{5}\cdot L^2 = 1215$ m^2 \Rightarrow L^2 = \dfrac{1215\cdot 5}{3}$ m^2 \Rightarrow L^2 = 405\cdot 5 $ m^2 \Rightarrow \\ \\ [/tex]
[tex]\Rightarrow L^2 = 81\cdot 5\cdot 5 $ m^2 \Rightarrow L^2 = 9^2\cdot 5^2$ m^2 \Rightarrow L = 9\cdot 5$ m \Rightarrow L=45 $ m \Rightarrow \\ \\ \Rightarrow\left\{ \begin{array}{c} L = 45$ m \\ l=\dfrac{3}{5}\cdot L$ m \end{array} \right \Rightarrow \left\{ \begin{array}{c} L = 45$ m \\ l=\dfrac{3}{5}\cdot 45 $ m\end{array} \right \Rightarrow \left\{ \begin{array}{c} L = 45$ m \\ l=27 $ m\end{array} \right | \\ \\[/tex]
[tex] P_{\boxed{~}} = 2\times(L+l)$ m \Rightarrow P_{\boxed{~}} = 2\times(45+27)$ m \Rightarrow\\ \\ \Rightarrow P = 2\times 72 $ m \Rightarrow \boxed{P_{\boxed{~}} = 144\text{ m}}[/tex]
