$\displaystyle{y}=\frac{{k}}{{x}}\usepackage{cmbright}\Huge\$

Substitute 30 for x and 9 for y: $\displaystyle{9}=\frac{{k}}{{30}}\usepackage{cmbright}\Huge\$

Multiply both sides by 30: $\displaystyle{9}\cdot{30}=\frac{{k}}{{30}}\cdot{30}\usepackage{cmbright}\Huge\$

$\displaystyle{270}={k}\usepackage{cmbright}\Huge\$