\[y = \frac{x^{4} - 9x^{2}}{(3 + x)(3 - x)} =\]
\[= \frac{x²(x^{2} - 9)}{- (x^{2} - 9)}\ \ \ \ \ \ \ \ \ \ x \neq \pm 3\ \]
\[y = - x²\]