(a+2)(a^2+4)(a^4+16)(2a-4)
=(a+2)2(a-2)(a^2+4)(a^4+16)
=2(a^2-4)(a^2+4)(a^4+16)
=2(a^4-16)(a^4+16)
=2(a^8-256)
=2a^8-512