x²+y²=x²-xy+y²+xy=3+5=8
2x²+xy+3y²=2x²-2xy+3y²+3xy=2(x²-xy)+3(y²+xy)=2×3+3×5=6+15=21
2x²-5xy-3y²=2x²-2xy-3y²-3xy=2(x²-xy)-3(y²+xy)=6-15=-9