You may have forgotten to substitute 2 for $\displaystyle{y}\usepackage{cmbright}\Huge\$ , and incorrectly treated $\displaystyle{3}{y}\usepackage{cmbright}\Huge\$ as if it were just 3. You can either distribute the 9 and then substitute the 2 for $\displaystyle{y}\usepackage{cmbright}\Huge\$ , or substitute first and then distribute. The correct answer is 36.