(2x-y-3)²
=(2x-y-3)(2x-y-3)
=2x*(2x-y-3)-y*(2x-y-3)-3*(2x-y-3)
=4x²-2xy-6x-2xy+y²+3y-6x+3y+9
=4x²+y²-4xy-12x+6y+9