When i try to factor this i get stuck. 16a2(b-4)2- 4a(b-4). (b-4)2 + (b-4) should be (b-4)(b-4+1) --> (b-4)(b-3). So i end up with. 16a2-4a(b-4)(b-3). I tried to factor further ending up with (b-4)(b-3)4a(4a-1) but that dosen't seem to be right according to my book. The book says 4a(b-4)(4ab-16a-1). I already came up with the 4a(b-4) so it seems they are getting (4ab-16a-1) from (4a-1)(b-3) which makes no sense.