\[- 12;\ - 10;\ - 8;\ldots \Longrightarrow d = 2;\ \ \ \ \]
\[S_{n} = \frac{2a_{1} + (n - 1)d}{2} \cdot n\]
\[S_{n} = \frac{2 \cdot ( - 12) + (n - 1) \cdot 2}{2} \cdot n =\]
\[= ( - 12 + n - 1) \cdot n =\]
\[= (n - 13)n = n^{2} - 13n\]
\[n^{2} - 13n = - 36\]
\[n^{2} - 13n + 36 = 0\]
\[n_{1} = 4;\ \ \ n_{2} = 9\]
\[Ответ:4\ или\ 9.\]