y=x^2-（m^2+4）x-2m^2-12
与x轴相交：
x^2-(m^2+4)x-2m^2-12=0
x1+x2=m^2+4
x1x2=-2m^2-12
图像在x轴上截得的线段的长是12
|x1-x2|=12
(x1-x2)^2=(x1+x2)^-4x1x2
=(m^2+4)^2+8m^2+48
=m^4+16m^2+64=144
(m^2+8)^2=12^2
m^2+8=12
m^2=4
m=±2