You may have forgotten to distribute the 4 on the left side. This would give you the incorrect equation $\displaystyle{12}{x}-{1}+{9}-{2}{x}={2}{x}+{5}+{8}{x}\usepackage{cmbright}\Huge\$ . The left side should be $\displaystyle{12}{x}-{4}+{9}-{2}{x}\usepackage{cmbright}\Huge\$ .