You may have forgotten to distribute the 4 to the $\displaystyle-{x}\usepackage{cmbright}\Huge\$ on the left side of the equation. The correct simplification is $\displaystyle{12}-{4}{x}+{7}{x}={4}{x}+{12}+{2}{x}\usepackage{cmbright}\Huge\$ . This will simplify further to $\displaystyle{12}+{3}{x}={6}{x}+{12}\usepackage{cmbright}\Huge\$ . This equation has the solution $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ .