All lengths are considered in feet.
\[? = 2\left[ {\left( {a - 6} \right) + \left( {b - 6} \right)} \right]\,\,\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\boxed{\,\,\,? = a + b\,\,}\]

\[\left( 1 \right)\,\,\,2\left( {a + b} \right) = 124\,\,\,\, \Rightarrow \,\,\,\,? = a + b\,\,\,{\text{unique}}\]
\[\left( 2 \right)\,\,\left( {a - 6} \right)\left( {b - 6} \right) = 600\,\,\,\,\left\{ \begin{gathered}
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( {16,66} \right)\,\,\,\,\, \Rightarrow \,\,\,\,{\text{?}}\,\,{\text{ = }}\,\,{\text{16 + 66}}\,\, \hfill \\
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( {26,36} \right)\,\,\,\,\, \Rightarrow \,\,\,\,{\text{?}}\,\,{\text{ = }}\,\,{\text{26 + 36}} \ne {\text{16 + 66}}\,\, \hfill \\
\end{gathered} \right.\]
This solution follows the notations and rationale taught in the GMATH method.
Regards,
Fabio.














