a'3+3a'2b+3b'2a-7b'3
=a'3+3a'2b-4b²a+7b²a-7b'3
=a(a²+3ab-4b²)+7b²(a-b)
=a(a-b)(a+4b)+7b²(a-b)
=(a²+4ab+7b²)(a-b)