You tried to combine $\displaystyle{7}{b}\usepackage{cmbright}\Huge\$ and $\displaystyle{{b}}^{{2}}\usepackage{cmbright}\Huge\$ as well as $\displaystyle-{3}\usepackage{cmbright}\Huge\$ and $\displaystyle-{2}{b}\usepackage{cmbright}\Huge\$ , but these are unlike terms and cannot be combined. Regroup by like terms to get $\displaystyle{5}{{b}}^{{3}}+{{b}}^{{2}}+{\left({7}{b}+-{2}{b}\right)}+{\left(-{3}+{6}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{5}{{b}}^{{3}}+{{b}}^{{2}}+{5}{b}+{3}\usepackage{cmbright}\Huge\$ .