Incorrect. The common denominator is $\displaystyle{3}{a}{b}\usepackage{cmbright}\Huge\$ . Rewrite each fraction so that it has this denominator:

$\displaystyle\frac{{7}}{{{3}{a}}}+\frac{{{9}{a}}}{{b}}=\frac{{7}}{{{3}{a}}}\cdot\frac{{b}}{{b}}+\frac{{{9}{a}}}{{b}}\cdot\frac{{{3}{a}}}{{{3}{a}}}\usepackage{cmbright}\Huge\$

Multiply, and then add the numerators. The correct answer is $\displaystyle{7}{b}\usepackage{cmbright}\Huge\$ .