Решение:
$$ \frac{36-u^2}{u^2-12u+36} = \frac{(6-u)(6+u)}{(u-6)^2} = \frac{-(u-6)(6+u)}{(u-6)(u-6)} = \frac{-(6+u)}{u-6} = -\frac{6+u}{u-6} = \frac{6+u}{6-u} $$
Ответ: $$ \frac{6+u}{6-u} $$