\[d = - 4;\ \ S_{12} = 936:\]
\[\frac{{2a}_{1} - 4 \cdot 11}{2} \cdot 12 = 336\]
\[\left( 2a_{1} - 44 \right) \cdot 6 = 336\]
\[{2a}_{1} - 44 = 56\ \ \ \]
\[{2a}_{1} = 100\ \ \]
\[a_{1} = 50.\]
\[a_{9} = a_{1} + 8d = 50 - 32 = 18.\]
\[Ответ:a_{1} = 50;\ \ \ a_{9} = 18.\ \]