(2a + b + 3c - 4b)^2= (2a + b + 3c - 4b) * (2a + b + 3c - 4b)= (2a + b + 3c - 4b)(2a + 3c + b - 4b)= 4a^2 + 6ac + 2ab - 8ab + b^2 - 4b^2 + 6ac + 9c^2 + 3bc - 12bc= 4a^2 + 12ac - 6ab - 3b^2 + 9c^2 - 9bc.
(2a + b + 3c - 4b)^2
= (2a + b + 3c - 4b) * (2a + b + 3c - 4b)
= (2a + b + 3c - 4b)(2a + 3c + b - 4b)
= 4a^2 + 6ac + 2ab - 8ab + b^2 - 4b^2 + 6ac + 9c^2 + 3bc - 12bc
= 4a^2 + 12ac - 6ab - 3b^2 + 9c^2 - 9bc.