7y(x-3y)^2-2(3y-x)^3 =(x-3y)^2[7y+2(x-3y)] =(x-3y)^2(7y+2x-6y) =(x-3y)^2(2x+y) 将2x+y=5,x-3y=1代入上式的右边,得 7y(x-3y)^2-2(3y-x)^3 =(x-3y)^2(2x+y) =1^2*5 =5
