-3(5x-7y)+6(2x-4y) 和 a-(2a-7b)+3(a-2b),其中a=-2,b=-100
=-15x+21y+12x-24y =a-2a+7b+3a-6b
=-3x-3y =2a+b
a=-2,b=-100
2a+b=2*(-2)+(-100)=-4-100=-104