You may have incorrectly simplified the equation to $\displaystyle{\left|{b}-{4}\right|}={18}\usepackage{cmbright}\Huge\$ , and then solved $\displaystyle{b}-{4}=-{18}\usepackage{cmbright}\Huge\$ or $\displaystyle{b}-{4}={18}\usepackage{cmbright}\Huge\$ . Actually it simplifies to $\displaystyle{\left|{b}-{4}\right|}=-{2}\usepackage{cmbright}\Huge\$ .