(b + 4)^2 - 2b(5b + 4) = b^2 + 8b + 16 - 10b^2 - 8b = -9b^2 + 16 = 16 - 9b^2 = (4 - 3b)(4 + 3b)
(b+4)^2 - 2b(5b+4) b^2 + 8b+16-10b^2 -8b -9b^2 +16