\(P = 6\ м\ 30\ см;\ \ a = 8b.\)
\[P = 2 \cdot (a + b) = 2(8b + b) = 2 \cdot 9b = 18b = 630\ см.\]
\[b = 630\ :18 = 35\ см.\]
\[a = 8b = 8 \cdot 35 = 280\ см.\]
\[S = ab = 280 \cdot 35 = 9800\ см^{2} = 98\ дм^{2}.\]
\[Ответ:98\ дм^{2}.\]