1.多项式a(x-y)+b(-y+x)-c(x-y)²各项的公因式是（_________)
2.a(x-y)²-3(y-x)=(x-y)(_______)
3.6(x-y)³+18(y-x)²=6(x-y)²(________)
4.先分解因式后求值：
18x²(x-2y)²-24xy(2y-x)²-6x(2y-x)³.其中x=-3,y=-1.