(x + 5y)² - (x - y)² = (x + 5y - x + y)(x + 5y + x - y) = 6y(2x + 4y) =
= 6y * 2 * (x + 2y) = 12y(x + 2y)
(x + 5y)² - (x - y)² = (x + 5y - x + y)(x + 5y + x - y) = 6y(2x + 4y) =
= 6y * 2 * (x + 2y) = 12y(x + 2y)