You may have correctly simplified the equation to $\displaystyle-{\left|{b}-{4}\right|}={2}\usepackage{cmbright}\Huge\$ , but then treated it as if it were $\displaystyle{\left|-{b}-{4}\right|}={2}\usepackage{cmbright}\Huge\$ , solving $\displaystyle-{b}-{4}={2}\usepackage{cmbright}\Huge\$ or $\displaystyle-{b}-{4}=-{2}\usepackage{cmbright}\Huge\$ . The correct way to simplify would be $\displaystyle{\left|{b}-{4}\right|}=-{2}\usepackage{cmbright}\Huge\$ .