(-8/x^2-9x)-(9/9x^3-9x^2)
I do not know if this is what you intended but it is what you asked for.
$$\\\frac{-8}{x^2}-9x-(\frac{9}{9}x^3-9x^2)\\\\=\frac{-8}{x^2}-9x-(x^3-9x^2)\\\\=\frac{-8}{x^2}-9x-x^3+9x^2\\\\$$