You left out the middle term. To square a 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}={6}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$ .