(3a²+2a-8)²-(a²-2a-8)²=[(3a²+2a-8)+(a²-2a-8)][(3a²+2a-8)-(a²-2a-8)]
=(4a²-16)(2a²+4a)
=8a(a+2)(a-2)(a+2)
=8a(a+2)²(a-2)