3A-2B=3*(y*y+x*y-x*x)-2(y*y-x*y+2*x*x) =2y^2+5xy-7x^2
3A=3Y2+3XY-3X22B=2Y2-2XY+4X2上减下3A-2B=Y2+5XY-7X2
答案见图~~~~~~~