[tex]l = L \div 2[/tex]
[tex]P=126 \: cm[/tex]
[tex]P=2(L+l)[/tex]
[tex]2(L+L \div 2) = 126 \: | \div 2[/tex]
[tex]L + L \div 2 = 63 \: | \times 2[/tex]
[tex]2L +L = 126[/tex]
[tex]3L = 126[/tex]
[tex]L = 126 \div 3[/tex]
[tex]L = 42 \: cm[/tex]
[tex]l = L \div 2[/tex]
[tex]l = 42 \div 2[/tex]
[tex]l = 21 \: cm[/tex]
[tex]A=L\:\times\:l=42\times21=882 \:{cm}^{2}[/tex]