You ignored the minus signs in the polynomials when adding. You need to rewrite the problem as $\displaystyle{6}{{b}}^{{3}}+{\left(-{2}{b}\right)}+{4}+{2}{{b}}^{{2}}+{5}{b}+{\left(-{1}\right)}\usepackage{cmbright}\Huge\$ before regrouping and combining like terms. The correct answer is $\displaystyle{6}{{b}}^{{3}}+{2}{{b}}^{{2}}+{3}{b}+{3}\usepackage{cmbright}\Huge\$ .