Correct. The bases are corresponding sides. The sides on the left are corresponding also. This gives you the proportion $\displaystyle\frac{{4}}{{10}}=\frac{{3}}{{n}}\usepackage{cmbright}\Huge\$ . Solve the equation:

$\displaystyle{4}{n}={30}\usepackage{cmbright}\Huge\$

$\displaystyle{n}={7.5}\usepackage{cmbright}\Huge\$