Question 529381
={{{(6x^2-7xy+y^2)+(-9x^2-9xy-y^2)+(x^2+xy-4y^2)}}}
={{{6x^2-7xy+y^2-9x^2-9xy-y^2+x^2+xy-4y^2)}}}
={{{6x^2+x^2-9x^2+y^2-y^2-4y^2-9xy-7xy+xy)}}} [Combine like terms]
={{{7x^2-9x^2+cross(y^2)-cross(y^2)-4y^2-16xy+xy)}}}
={{{-2x^2-4y^2-15xy)}}}
={{{-(2x^2+4y^2+15xy)}}}