(2a+b)^2-(a-2b)^2
(x+y)^2-(y-z)^2

(2a+b)^2-(a-2b)^2 = (4а^2 + 4ab + b^2) - (a^2 - 4ab + 4b^2) = 4а^2 + 4ab + b^2 - a^2 + 4ab - 4b^2 = 8ab -3b^2