<s>[INST] How do I simplify this expression about factorization? I am trying to simplify this
$$\frac{9x^2 - x^4}  {x^2 - 6x +9}$$
The solution is 
$$\frac{-x^2(x +3)}{x-3} = \frac{-x^3 - 3x^2}{x-3} $$
I have done $$\frac{x^2(9-x^2)}{(x-3)(x-3)} = \frac{x^2(3-x)(3+x)}{(x-3)(x-3)} $$
but I do not find a way to simplify 
How can I simplify to get the answer?
I have to use
$${(a +b)(a -b)} = {a^2 - b^2} $$
$${(a +b)^2} = {a^2 + 2ab+b^2} $$
$${(a -b)^2} = {a^2 - 2ab+b^2} $$
 [/INST] Notice that $(3-x)=-(x-3)$. Simplify accordingly.
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