You have reversed the placement of the minus and plus signs. To square the binomial, you can use $\displaystyle{{\left({a}-{b}\right)}}^{{2}}={{a}}^{{2}}-{2}{a}{b}+{{b}}^{{2}}\usepackage{cmbright}\Huge\$ . In this case, $\displaystyle{a}={8}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$ .