$\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\$ , so $\displaystyle{{\left({4}{x}+{7}\right)}}^{{2}}={{\left({4}{x}\right)}}^{{2}}+{2}{\left({4}{x}\right)}{\left({7}\right)}+{{7}}^{{2}}={16}{{x}}^{{2}}+{56}{x}+{49}\usepackage{cmbright}\Huge\$ .