已知A=x的平方-7x-2,B=负2x的平方+4x-3,求（1）A-B（2）2A+B（3）2A-3B

A= X^2-7X-2
B= -2X^2+4X-3
1.A-B= X^2-7X-2+2X^2-4X+3
= 3X^2-11X+1
2.2A+B = 2X^2-14X-4 -2X^2+4X-3
= 4X^2-10X-7
3.2A-3B = 2X^2-14X-4+6X^2-12X+9
= 8X^2-26X+5