\[S_{12} = \frac{2a_{1} + 11d}{2} \cdot 12 =\]
\[= \left( 2a_{1} + 11d \right) \cdot 6.\]
\[a_{1} = 16,5;\ \ d = - 1,5:\]
\[S_{12} = (2 \cdot 16,5 - 11 \cdot 1,5) \cdot 6 =\]
\[= (33 - 16,5) \cdot 6 = 16,5 \cdot 6 =\]
\[= 99.\]