\[(2b - 1)^{2} - (b + 2)^{2} =\]
\[= \left( 2b - 1 - (b + 2) \right)\left( 2b - 1 + (b + 2) \right) =\]
\[= (2b - 1 - b - 2)(2b - 1 + b + 2) =\]
\[= (b - 3)(3b + 1).\]