4x^2 - 20xy + 25y^2 - 9y^2 = (2x - 5y)^2 - (3y)^2 = (2x - 5y + 3y)(2x - 5y - 3y) = (2x - 2y)(2x - 8y) = 2(x - y)(2x - 8y)
4x^2 - 20xy + 25y^2 - 9y^2 = (2x - 5y)^2 - (3y)^2 = (2x - 5y + 3y)(2x - 5y - 3y) = (2x - 2y)(2x - 8y) = 2(x - y)(2x - 8y)