\[9x^{2} - (c + 3x)(c - 3x) =\]
\[= 9x^{2} - \left( c^{2} - (3x)^{2} \right) = \ \]
\[= 9x^{2} - c^{2} + 9x^{2} =\]
\[= 18x^{2} - c^{2}\text{.\ }\]