64-x³-3x²+9x+12=0
4³-x³-3(x²-3x-4)=0
(4-x)(16+4x+x²)-3(x-4)(x+1)=0
(4-x)(16+4x+x²+3x+3)=0
(x-4)(x²+7x+19)=0
当x²+7x+19=0时,△=7²-4×19＜0,无解；
当x-4=0时,得：x=4 .