$\begin{array}{l}\phantom{\phantom{;}3f^{2}+4;}{\phantom{;}6f^{2}-f\phantom{;}-8\phantom{;}\phantom{;}}\\\phantom{;}3f^{2}+4\overline{\smash{)}\phantom{;}18f^{4}-3f^{3}\phantom{-;x^n}-5f\phantom{;}-1\phantom{;}\phantom{;}}\\\phantom{\phantom{;}3f^{2}+4;}\underline{-18f^{4}\phantom{-;x^n}-24f^{2}\phantom{-;x^n}\phantom{-;x^n}}\\\phantom{-18f^{4}-24f^{2};}-3f^{3}-24f^{2}-5f\phantom{;}-1\phantom{;}\phantom{;}\\\phantom{\phantom{;}3f^{2}+4-;x^n;}\underline{\phantom{;}3f^{3}\phantom{-;x^n}+4f\phantom{;}\phantom{-;x^n}}\\\phantom{;\phantom{;}3f^{3}+4f\phantom{;}-;x^n;}-24f^{2}-f\phantom{;}-1\phantom{;}\phantom{;}\\\phantom{\phantom{;}3f^{2}+4-;x^n-;x^n;}\underline{\phantom{;}24f^{2}\phantom{-;x^n}+32\phantom{;}\phantom{;}}\\\phantom{;;\phantom{;}24f^{2}+32\phantom{;}\phantom{;}-;x^n-;x^n;}-f\phantom{;}+31\phantom{;}\phantom{;}\\\end{array}$

$6f^{2}-f-8+\frac{-f+31}{3f^2+4}$