You may have forgotten to substitute 6 for $\displaystyle{x}\usepackage{cmbright}\Huge\$ , and incorrectly treated $\displaystyle{2}{x}\usepackage{cmbright}\Huge\$ as if it were just 2. You can either distribute the 4 and then substitute the 6 for $\displaystyle{x}\usepackage{cmbright}\Huge\$ , or substitute first and then distribute.