9x^2 - y−3xy - 3xy−3x^2 = 9x^2 - y2−6xy+9x2y^2 - 6xy + 9x^2y2−6xy+9x2 9x^2 - y^2 + 6xy - 9x^26xy - y^2
9x^2 - y−3xy - 3xy−3x^2 = 9x^2 - y2−6xy+9x2y^2 - 6xy + 9x^2y2−6xy+9x2 9x^2 - y^2 + 6xy - 9x^2
6xy - y^2