解3x²y-{x²z-[3x²y+(-xyz)-3x²z]}=3x²y-[x²z-(3x²y-xyz-3x²z)]=3x²y-(x²z-3x²y+xyz+3x²z)=3x²y-(4x²z-3x²y+xyz)=3x²y-4x²z+...