Incorrect. Factor both terms, find their GCF, and factor it out of the expression.

$\displaystyle{18}{{a}}^{{4}}{{b}}^{{2}}={2}\cdot{3}\cdot{3}\cdot{a}\cdot{a}\cdot{a}\cdot{a}\cdot{b}\cdot{b}\usepackage{cmbright}\Huge\$

$\displaystyle{30}{{a}}^{{2}}{{b}}^{{3}}={2}\cdot{3}\cdot{5}\cdot{a}\cdot{a}\cdot{b}\cdot{b}\cdot{b}\usepackage{cmbright}\Huge\$

Multiply the common factors to get the GCF.