A+B=2x²-3xy+2y²+2x²+xy-3y²=4x²-2xy-y²
A-(B-2A)=3A-B=3(2x²-3xy+2y²)-(2x²+xy-3y²)
=6x²-9xy+6y²-2x²-xy+3y²=4x²-10xy+9y²