It seems that you divided by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ instead of multiplying both sides of the equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ . Dividing $\displaystyle{\left(-\frac{{1}}{{3}}\right)}{x}\usepackage{cmbright}\Huge\$ by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ will not leave you with x, and you must do the same thing to both sides.