a^4-16b^4=(a^2+4b^2)(a^2-4b^2)
a^2-4b^2=(a+2b)(a-2b)
(a^4-16b^4)÷(a^2+4b^2)÷(2b-a)
=(a^2-4b^2)÷(2b-a)
=-a-2b