$$f ( x ) = \frac { ( 2 x ) ^ { \prime } ( x ^ { 2 } - 36 ) - ( 2 x ) ( x ^ { 2 } - 36 ) ^ { \prime } } { ( x ^ { 2 } - 36 ) ^ { 2 } } =$$
$f=-\frac{2\left(x^{2}+36\right)}{x\left(x^{2}-36\right)^{2}}$
$x\neq 0\text{ and }|x|\neq 6$