Дано: \( c_1 = -5 \) и \( c_{n+1} = c_n - 2 \).
Последовательно вычисляем:
\[c_2 = c_1 - 2 = -5 - 2 = -7\]
\[c_3 = c_2 - 2 = -7 - 2 = -9\]
\[c_4 = c_3 - 2 = -9 - 2 = -11\]
\[c_5 = c_4 - 2 = -11 - 2 = -13\]
\[c_6 = c_5 - 2 = -13 - 2 = -15\]
\[c_7 = c_6 - 2 = -15 - 2 = -17\]
Ответ: -17