\[\frac{{(a - b)^{2}}^{\backslash 2}}{18b} - \frac{{(a - b)^{2}}^{\backslash 3}}{12b} + \frac{a^{2} - b^{2}}{36b} =\]
\[= \frac{- a^{2} + 2ab - b^{2} + a^{2} - b^{2}}{36b} =\]
\[= \frac{2ab - 2b^{2}}{36b} = \frac{2b(a - b)}{36b} =\]
\[= \frac{a - b}{18}\]