Write a proportion, cross multiply, and divide to isolate the variable.

$\displaystyle\frac{{n}}{{3}}=\frac{{12}}{{9}}\usepackage{cmbright}\Huge\$

$\displaystyle{9}{n}={36}\usepackage{cmbright}\Huge\$

$\displaystyle{n}=\frac{{36}}{{9}}\usepackage{cmbright}\Huge\$

$\displaystyle{n}={4}\usepackage{cmbright}\Huge\$