When changing the subtraction to addition of the opposite, you forgot to change the negative signs to positive in the second polynomial. You should add $\displaystyle{\left(-{13}{{f}}^{{3}}+{9}{{f}}^{{2}}+{7}{f{-}}{16}\right)}+{\left({8}{{f}}^{{3}}-{5}{{f}}^{{2}}-{9}{f{+}}{4}\right)}\usepackage{cmbright}\Huge\$ .