(3a-4b)(7a-8b)+(11a-12b)(8b-7a)
=(3a-4b)(7a-8b)-(11a-12b)(7a-8b)
=(3a-4b-11a+12b)(7a-8b)
=(8b-8a)(7a-8b)
=8(b-a)(7a-8b)