x^3/x^2-4/x^3-9x^2/x^2+7x-18
factor everything so you know what to cancel out;
Also, take the bottom "fraction"; turn it over, spank it, and multiply it to the top "fraction"; like this:
xxx (x+9)(x-2)
--------- * ----------
(x-2)(x+2) xx (x-9)
Now cancel like terms top to bottom
x(x+9)
----------
(x+2) (x-9)