Сократите дробь: (7+6c-c^2)/(21-3c).

\[\frac{7 + 6c - c^{2}}{21 - 3c} = \frac{- \left( c^{2} - 6c - 7 \right)}{- 3(c - 7)} =\]

\[= \frac{(c - 7)(c + 1)}{3(c - 7)} = \frac{c + 1}{3}\]

\[c^{2} - 6c - 7 = 0\]

\[c_{1} + c_{2} = 6;\ \ \ c_{1} \cdot c_{2} = - 7\]

\[c_{1} = 7;\ \ \ c_{2} = - 1.\]