2xy-x²-y²
=-(x²-2xy+y²)
=-(x-y)²
4xy²-4x²y-y³
=y(4xy-4x²-y²)
=-y(4x²-4xy+y²)
=-y(2x-y)²
3-6x+3x²
=3(1-2x+x²)
=3(1-x)²
-a+2a²-a³
=-a(1-2a+a²)
=-a(1-a)²