Correct. Use the joint variation equation $\displaystyle{y}={k}{x}{z}\usepackage{cmbright}\Huge\$ to find k.

$\displaystyle{3},{600}={k}{\left({6},{000}\right)}{\left({5}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{0.12}={k}\usepackage{cmbright}\Huge\$

Use this value for k to find the interest charged on $8,000 for 4 years:

$\displaystyle{I}={0.12}{\left({8},{000}\right)}{\left({4}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{I}={3},{840}\usepackage{cmbright}\Huge\$