\[- 21;\ - 18;\ - 15;\ldots\]
\[a_{1} = - 21;\ \ a_{2} = - 18;\ \]
\[d = a_{2} - a_{1} = - 18 + 21 = 3;\]
\[S_{20} = \frac{2a_{1} + 19d}{2} \cdot 20 =\]
\[= \left( 2 \cdot ( - 21) + 3 \cdot 19 \right) \cdot 10 =\]
\[= ( - 42 + 57) \cdot 10 = 150.\]
\[Ответ:150.\]