\[\frac{b(b - 3)}{b^{2} - 6b + 9} = \frac{b(b - 3)}{(b - 3)^{2}} = \frac{b}{b - 3};\]
\[\frac{b}{b - 3} = 0;\ \ b \neq 3\]
\[b = 0.\]
\[Ответ:при\ b = 0.\]