Rewrite as adding the opposite: $\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\$

Regroup to match like terms: $\displaystyle{\left(-{13}{{f}}^{{3}}+{8}{{f}}^{{3}}\right)}+{\left(+{9}{{f}}^{{2}}+-{5}{{f}}^{{2}}\right)}+{\left({7}{f{+}}-{9}{f}\right)}+{\left(-{16}+{4}\right)}\usepackage{cmbright}\Huge\$

Combine like terms: $\displaystyle-{5}{{f}}^{{3}}+{4}{{f}}^{{2}}-{2}{f{-}}{12}\usepackage{cmbright}\Huge\$