7y(x-3y)^2-2(3y-x)^3
=7y(3y-x)^2-2(3y-x)^3
=(3y-x)^2[7y-2(3y-x)]
=(3y-x)^2(y+2x)
当2x+y=6,x-3y=1时
原式=(-1)^2*6=6