You may have reversed the variables when substituting their values into the expression. $\displaystyle-{2}\usepackage{cmbright}\Huge\$ should be substituted in for both occurrences of a and 4 should be substituted in for b, to give you $\displaystyle{{\left(-{2}\right)}}^{{2}}+{6}{\left(-{2}\right)}{\left({4}\right)}\usepackage{cmbright}\Huge\$ .