x=根号7-1,
∴x+1=√7
(x+1)²=7
即x²+2x+1=7
x²+2x=6
3x³+12x²-6x-20
=3x(x²+2x)+6x²-6x-20
=3x×6+6x²-6x-20
=6x²+12x-20
=6(x²+2x)-20
=6×6-20
=16