\[c_{1} = - 6;\ \ c_{9} = 6:\]
\[c_{9} = c_{1} + 8d\]
\[6 = - 6 + 8d\]
\[8d = 12\]
\[d = \frac{12}{8} = 1,5.\]
\[c_{n} = 39:\]
\[c_{n} = c_{1} + d(n - 1)\]
\[- 6 + 1,5(n - 1) = 39\]
\[1,5n - 1,5 = 45\]
\[1,5n = 46,5\]
\[n = 31.\]
\[Ответ:\ да,\ является;c_{31} = 39.\]