3a³-6a²-14a+20
=3a³-6a²-24a+10a+20
=3a(a²-2a-8)+10(a+2)
=3a(a-4)(a+2)+10(a+2)
=(a+2)(3a²-12a+10)=0
则a+2=0 3a²-12a+10=0
a=-2 或 (6-√6)/3 或 (6+√6)/3