(x-y）（a-2b+3c）-3（y-x)(2b-a-3c）
=(x-y)(a-2b+3c)+3(x-y)(2b-a-3c)
=(x-y)(a-2b+3c+6b-3a-9c)
=(x-y)(-2a+4b-6c)
=-2(x-y)(a-2b+3c)