Incorrect. To add rational expressions with unlike denominators, first find the least common denominator.

$\displaystyle{27}{{y}}^{{2}}={3}\cdot{3}\cdot{3}\cdot{y}\cdot{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{18}{{y}}^{{3}}={2}\cdot{3}\cdot{3}\cdot{y}\cdot{y}\cdot{y}\usepackage{cmbright}\Huge\$ , so the LCD $\displaystyle={2}\cdot{3}\cdot{3}\cdot{3}\cdot{y}\cdot{y}\cdot{y}={54}{{y}}^{{3}}\usepackage{cmbright}\Huge\$ .

Rewrite each fraction with the common denominator of $\displaystyle{54}{{y}}^{{3}}\usepackage{cmbright}\Huge\$ . Add the numerators and simplify if possible.