Ответ (RU):
$$\frac{a^{-7}}{(a^{-2}a^3)^{-3}} = \frac{a^{-7}}{(a^{-2+3})^{-3}} = \frac{a^{-7}}{(a^{1})^{-3}} = \frac{a^{-7}}{a^{-3}} = a^{-7-(-3)} = a^{-7+3} = a^{-4} = \frac{1}{a^4}$$