Упростите выражение (a/a+4−a/a−4):a/a^2−16

Решите задачу:

$( \frac{a}{a + 4} - \frac{a}{a - 4} ) \div \frac{a}{ {a}^{2} - 16 } = \frac{a(a - 4) - a(a + 4)}{(a + 4)(a - 4)} \div \frac{a}{ { a}^{2} - 16} = \frac{ {a}^{2} - 4a - {a}^{2} - 4a }{(a + 4)(a - 4)} \div \frac{ {a}}{ {a}^{2} - 16 } = \frac{ - 8a}{(a + 4)(a - 4)} \div \frac{a}{ {a}^{2} - 16} = - \frac{8a}{(a + 4)(a - 4)} \times \frac{ {a}^{2} - 16}{ {a}^{} } = - \frac{8a}{(a + 4)(a - 4)} \times (a - 4)(a + 4) = - 8$