\[9a + 9b - ax^{2} - bx^{2} =\]
\[= 9 \cdot (a + b) - x^{2}(a + b) =\]
\[= (a + b)\left( 9 - x^{2} \right) =\]
\[= (a + b)(3 - x)(3 + x)\]