\[9a^{2} - ab + \frac{1}{36}b^{2} = \left( 9a - \frac{1}{6}b \right)^{2} =\]
\[= \left( 9a - \frac{1}{6}b \right)\left( 9a - \frac{1}{6}b \right)\text{.\ }\]