(15-2x-x²)/(x²+x-12)
=-(x²+2x-15)/(x²+x-12)
=-(x+5)(x-3)/[(x+4)(x-3)]
=-(x+5)/(x-3)
(x²+x-12)分之(15-2x-x²)=负(x-3)分之(x+5)