You may have missed the negative signs on $\displaystyle-{42}\usepackage{cmbright}\Huge\$ and $\displaystyle-{28}\usepackage{cmbright}\Huge\$ . To correctly do the first step, $\displaystyle{15}+{\left(-{42}\right)}\usepackage{cmbright}\Huge\$ , you must find $\displaystyle{42}-{15}\usepackage{cmbright}\Huge\$ and make the result negative. To correctly do the last step, you must rewrite subtraction of $\displaystyle-{28}\usepackage{cmbright}\Huge\$ as addition of 28.