Incorrect. First rewrite each term using $\displaystyle{18}{a}{b}\usepackage{cmbright}\Huge\$ as the denominator:
$\displaystyle\frac{{1}}{{{6}{a}}}+\frac{{1}}{{{9}{b}}}-\frac{{1}}{{{18}{a}{b}}}=\frac{{{1}}}{{{6}{a}}}\cdot\frac{{{3}{b}}}{{{3}{b}}}+\frac{{1}}{{{9}{b}}}\cdot\frac{{{2}{a}}}{{{2}{a}}}-\frac{{1}}{{{18}{a}{b}}}\usepackage{cmbright}\Huge\$

Simplify each and then combine the numerators.