2x+y=6
x-3y=1
x(x-3y)²-(x+y)(3y-x)³
=x(x-3y)²+(x+y)(x-3y)³
=(x-3y)²[x+(x+y)(x-3y)]
带入 x-3y=1得
=x+x+y
=2x+y
=6