$\begin{array}{l}\phantom{\phantom{;}2a\phantom{;}-3;}{\phantom{;}8a\phantom{;}+12\phantom{;}\phantom{;}}\\\phantom{;}2a\phantom{;}-3\overline{\smash{)}\phantom{;}16a^{2}\phantom{-;x^n}-81\phantom{;}\phantom{;}}\\\phantom{\phantom{;}2a\phantom{;}-3;}\underline{-16a^{2}+24a\phantom{;}\phantom{-;x^n}}\\\phantom{-16a^{2}+24a\phantom{;};}\phantom{;}24a\phantom{;}-81\phantom{;}\phantom{;}\\\phantom{\phantom{;}2a\phantom{;}-3-;x^n;}\underline{-24a\phantom{;}+36\phantom{;}\phantom{;}}\\\phantom{;-24a\phantom{;}+36\phantom{;}\phantom{;}-;x^n;}-45\phantom{;}\phantom{;}\\\end{array}$

$8a+12+\frac{-45}{2a-3}$