Let x represent the unknown side. Set up a proportion of corresponding sides:

$\displaystyle\frac{{4}}{{x}}=\frac{{6}}{{12}}\usepackage{cmbright}\Huge\$

$\displaystyle{6}{x}={4}\cdot{12}={48}\usepackage{cmbright}\Huge\$

$\displaystyle{x}={8}\usepackage{cmbright}\Huge\$