x^2 + 2xy + 3x - 3y^2 + 9y = \\ (x+3y)^2 - 4xy -12y^2 + 3 \cdot (x+3y) = \\ (x+3y)^2 - 4y \cdot (x+3y) + 3 \cdot (x+3y) = \\ (x+3y) \cdot (x+3y-4y+3) = \\ (x+3y) \cdot (x-y+3)