-3(2x²-xy)+4(x²+xy-6)
=-6x²+3xy+4x²+4xy-24
=-2x²+7xy-24
-8m²-【4m-2m²-（3m-m²-7）-8】
=-8m²-4m+2m²+3m-m²-7+8
=-7m²-m+1