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