\[7 + 3 \cdot \left( - k - 3 \cdot (k + 5) \right) =\]
\[= 5 \cdot (7 - 2k) + k\]
\[7 + 3 \cdot ( - k - 3k - 15) =\]
\[= 35 - 10k + k\]
\[7 - 3k - 9k - 45 - 35 + 10k - k = 0\]
\[- 3k - 73 = 0\]
\[3k = - 73\]
\[k = - \frac{73}{3}\]
\[k = - 24\frac{1}{3}.\]
\[Ответ:k = - 24\frac{1}{3}.\]