Ответ: 9
\[\sqrt{c^2 + 14ct + 49t^2} = \sqrt{(c + 7t)^2} = |c + 7t|\]
\[\left| 8\frac{5}{12} + 7 \cdot \frac{1}{12} \right| = \left| \frac{101}{12} + \frac{7}{12} \right| = \left| \frac{108}{12} \right| = |9| = 9\]