1.2（x-y）²-x+y
=(x-y)(2x-2y-1)
2.81-9x²
=9(9-x^2)
=9(3+x)(3-x)
3.-2a³+12a²-18a
=-2a(a^2-6a+9)
=-2a(a-3)^2
4.x²-5x-6
=(x-6)(x+1)