You probably substituted correctly to get $\displaystyle-{6}{\left(\frac{{3}}{{4}}\right)}\usepackage{cmbright}\Huge\$ , and then multiplied to get $\displaystyle-\frac{{18}}{{4}}\usepackage{cmbright}\Huge\$ . But then you may have multiplied the reciprocal of $\displaystyle-\frac{{18}}{{4}}\usepackage{cmbright}\Huge\$ by 3, instead of multiplying $\displaystyle-\frac{{18}}{{4}}\usepackage{cmbright}\Huge\$ by the reciprocal of 3.