\[a_{1} = 3,8;\ \ \ \ \ d = - 1,4:\]
\[S_{18} = \frac{2a_{1} + 17d}{2} \cdot 18 =\]
\[= 9 \cdot \left( 2 \cdot 3,8 + 17 \cdot ( - 1,4) \right) =\]
\[= (7,8 - 23,8) \cdot 9 = - 145,8.\]
\[Ответ:\ - 145,8.\]