You may have multiplied 3 feet by $\displaystyle\frac{{{3}\ \text{ inches}}}{{{1}\ \text{ foot}}}\usepackage{cmbright}\Huge\$ and then added 6 inches. You need to multiply 3 feet by $\displaystyle\frac{{{12}\ \text{ inches}}}{{{1}\ \text{ foot}}}\usepackage{cmbright}\Huge\$ and then add 6 inches.