A+B=2x^2-8x+3 +3x^2-4x-5=5x^2-12x-2
2分之A+B =5/2x^2-6x-1
3A-2B=3(2x^2-8x+3 )-2(3x^2-4x-5)=6x^2-24x+9-6x^2+8x+10=-16x+19