\[9x^{2} - (x - 1)^{2} =\]
\[= \left( 3x - (x - 1) \right)\left( 3x + (x - 1) \right) =\]
\[= (3x - x + 1)(3x + x - 1) =\]
\[= (2x + 1)(4x - 1)\]