Do the operations and simplify a + 9 / 16 - a^ 2 * 9a - 36 / 9a + 81?

(a + 9)/(16 - a^2) * (9a - 36)/(9a + 81)
= (a + 9)/(4 + a)(4 - a) * 9(a - 4)/9(a + 9) (factorization)
= (a + 9)/-1(4 + a)(a - 4) * 9(a - 4)/9(a + 9) (caancel out a + 9 , a - 4)
= 1/-1(4 + a) * 9/9
= -1/(4 + a) * 1
= -1/(4 + a)

(a + 9) (9) (a - 4)
--------------------------------
(4 - a) (4 + a) (9) (a + 9)

(a - 4)
-------------------
- (a - 4) (4 + a)

- 1 / (4 + a)