You may have forgotten to distribute the 3 on the left side. This would give you the incorrect equation $\displaystyle{12}{x}-{1}+{9}-{x}={2}{x}+{8}+{9}{x}\usepackage{cmbright}\Huge\$ . The left side should be $\displaystyle{12}{x}-{3}+{9}-{x}\usepackage{cmbright}\Huge\$ .