\[P = 5\ м\ 6\ дм;\ \ a = 6b.\]
\[P = 2 \cdot (a + b) = 2(6b + b) =\]
\[= 56\ дм.\]
\[2 \cdot 7b = 56\]
\[14b = 56\]
\[b = 4\ дм.\]
\[a = 6b = 6 \cdot 4 = 24\ дм.\]
\[S = ab = 24 \cdot 4 = 96\ дм^{2}.\]
\[Ответ:96\ дм^{2}.\]