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}={t}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={4}\usepackage{cmbright}\Huge\$ .