\[- 21;\ - 18;\ - 15;\ldots\]
\[a_{1} = - 21;\ \ a_{2} = - 18;\]
\[d = - 18 + 21 = 3;\ \]
\[a_{20} = a_{1} + 19d = - 21 + 19 \cdot 3 =\]
\[= - 21 + 57 = 36;\]
\[S_{20} = \frac{a_{1} + a_{20}}{2} \cdot 20 =\]
\[= ( - 21 + 36) \cdot 10 = 15 \cdot 10 = 150.\]
\[Ответ:150.\]