You have simply doubled each term. 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}={4}{x}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={7}\usepackage{cmbright}\Huge\$ .