You incorrectly tried to combine $\displaystyle{6}{{b}}^{{3}}\usepackage{cmbright}\Huge\$ and $\displaystyle{2}{{b}}^{{2}}\usepackage{cmbright}\Huge\$ , but these are unlike terms and cannot be combined. Regroup by like terms to get $\displaystyle{6}{{b}}^{{3}}+{2}{{b}}^{{2}}+{\left(-{2}{b}+{5}{b}\right)}+{\left({4}+{\left(-{1}\right)}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{6}{{b}}^{{3}}+{2}{{b}}^{{2}}+{3}{b}+{3}\usepackage{cmbright}\Huge\$ .