\[\frac{c^{2} - 6c}{36 - c^{2}} = - \frac{c(6 - c)}{(6 - c)(6 + c)} =\]
\[= - \frac{c}{6 + c};\ \]
\[c = 1,2:\]
\[- \frac{1,2}{6 + 1,2} = - \frac{1,2}{7,2} = - \frac{12}{72} = - \frac{1}{6}.\]