\[(c - 5)(c - 1) - (c - 6)^{2} =\]
\[= c^{2} - c - 5c + 5 - \left( c^{2} - 12c + 36 \right) =\]
\[= c^{2} - 6c + 5 - c^{2} + 12c - 36 =\]
\[= 6c - 31.\]