soluzione Scomporre     15x - 5y - 9x2 + 6xy - y2 = = (15x - 5y) - (9x2 - 6xy + y2) = = 5(3x - y) - (3x -y)2 = = (3x - y) [5 - (3x -y)] = = (3x - y) (5 - 3x + y) = = -(3x - y) (3x - y - 5)