You may have mistakenly combined the $\displaystyle{3}{x}\usepackage{cmbright}\Huge\$ and $\displaystyle-{2}{x}\usepackage{cmbright}\Huge\$ on the left side, as well as the 9 and $\displaystyle-{1}\usepackage{cmbright}\Huge\$ , before distributing the 4. This is the incorrect order of operations. Multiplication comes before subtraction. The left side should first be simplified to $\displaystyle{12}{x}-{4}+{9}-{2}{x}\usepackage{cmbright}\Huge\$ .