You probably substituted correctly to get $\displaystyle-{8}{\left(-\frac{{1}}{{3}}\right)}\usepackage{cmbright}\Huge\$ , and then multiplied to get $\displaystyle\frac{{8}}{{3}}\usepackage{cmbright}\Huge\$ . But then you may have multiplied the reciprocal of $\displaystyle\frac{{8}}{{3}}\usepackage{cmbright}\Huge\$ by $\displaystyle-{4}\usepackage{cmbright}\Huge\$ , instead of multiplying $\displaystyle\frac{{8}}{{3}}\usepackage{cmbright}\Huge\$ by the reciprocal of $\displaystyle-{4}\usepackage{cmbright}\Huge\$ .