$\begin{array}{l}\phantom{\phantom{;}x\phantom{;}+3;}{-x^{3}+5x^{2}-15x\phantom{;}+45\phantom{;}\phantom{;}}\\\phantom{;}x\phantom{;}+3\overline{\smash{)}-x^{4}+2x^{3}\phantom{-;x^n}\phantom{-;x^n}-12\phantom{;}\phantom{;}}\\\phantom{\phantom{;}x\phantom{;}+3;}\underline{\phantom{;}x^{4}+3x^{3}\phantom{-;x^n}\phantom{-;x^n}\phantom{-;x^n}}\\\phantom{\phantom{;}x^{4}+3x^{3};}\phantom{;}5x^{3}\phantom{-;x^n}\phantom{-;x^n}-12\phantom{;}\phantom{;}\\\phantom{\phantom{;}x\phantom{;}+3-;x^n;}\underline{-5x^{3}-15x^{2}\phantom{-;x^n}\phantom{-;x^n}}\\\phantom{;-5x^{3}-15x^{2}-;x^n;}-15x^{2}\phantom{-;x^n}-12\phantom{;}\phantom{;}\\\phantom{\phantom{;}x\phantom{;}+3-;x^n-;x^n;}\underline{\phantom{;}15x^{2}+45x\phantom{;}\phantom{-;x^n}}\\\phantom{;;\phantom{;}15x^{2}+45x\phantom{;}-;x^n-;x^n;}\phantom{;}45x\phantom{;}-12\phantom{;}\phantom{;}\\\phantom{\phantom{;}x\phantom{;}+3-;x^n-;x^n-;x^n;}\underline{-45x\phantom{;}-135\phantom{;}\phantom{;}}\\\phantom{;;;-45x\phantom{;}-135\phantom{;}\phantom{;}-;x^n-;x^n-;x^n;}-147\phantom{;}\phantom{;}\\\end{array}$

$-x^{3}+5x^{2}-15x+45+\frac{-147}{x+3}$