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

Substitute 9 for x and 25 for y: $\displaystyle{25}=\frac{{k}}{{9}}\usepackage{cmbright}\Huge\$

Multiply both sides by 9: $\displaystyle{25}\cdot{9}=\frac{{k}}{{9}}\cdot{9}\usepackage{cmbright}\Huge\$

$\displaystyle{225}={k}\usepackage{cmbright}\Huge\$