（2x-6）/(4-4x+x²)×（x²+x-6）/（12-4x）/（x+3）=[2(x-3)/(x-2)²]×(x+3)(x-2)×1/4(3-x)×1/(x+3)=-[2(x-3)/(x-2)²]×(x+3)(x-2)×1/4(x-3)×1/(x+3)=1/2(x-2)