\[- 7;\ - 6;\ldots\]
\[S_{n} = - 25;\ \ \ \ \ \ d = 1\]
\[S_{n} = \frac{2a_{1} + (n - 1)d}{2} \cdot n\]
\[\frac{2 \cdot ( - 7) + (n - 1) \cdot 1}{2} \cdot n = - 25\]
\[\frac{- 14 + n - 1}{2} \cdot n = - 25\]
\[\frac{(n - 15) \cdot n}{2} = - 25\]
\[(n - 15)n = - 50\]
\[n^{2} - 15n + 50 = 0\]
\[n_{1} = 10;\ \ n_{2} = 5.\]
\[Ответ:\ \ 5\ или\ 10.\]