Use the inverse variation equation $\displaystyle{y}=\frac{{k}}{{x}}\usepackage{cmbright}\Huge\$ ; $\displaystyle{36}=\frac{{k}}{{6}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}={216}\usepackage{cmbright}\Huge\$ .

Substitute 54 and 4.

$\displaystyle{54}=\frac{{k}}{{4}}\usepackage{cmbright}\Huge\$ also yields $\displaystyle{k}={216}\usepackage{cmbright}\Huge\$ , so this is an inverse relationship.