\[15c - 5(c + 3)^{2} =\]
\[= 15c - 5\left( c^{2} + 6c + 9 \right) =\]
\[= 15c - 5c^{2} - 30c - 45 =\]
\[= - 5c^{2} - 15c - 45.\]