由2x+y=3,x-3y=1解得x=10/7，y=1/7所以14y(x-3y)的2次方-4(3y-x)的3次方 =(x-3y)^2*(14y-12y+4x) =(10/7-3/7)^2*(2/7+40/7) =1*6 =6
