(16-a²)／(a²+8a+16) ÷ (a-4)／(2a+8) × (a-2)／(a+2)
= -(a²-16)／(a²+8a+16) × (2a+8)／(a-4) × (a-2)／(a+2)
= -(a+4)(a-4)／(a+4)² × 2(a+4)／(a-4) × (a-2)／(a+2)
= - 2 (a-2)／(a+2)