原式=3a⁶+12a⁴-（a³+2a）+12a²-4
=3a⁶+12a⁴+12a²-2
=3a³（a³+2a+2a）+12a²-2
=3（-2a-2）（-2+2a）+12a²-2
=12-12a²+12a²-2
=10．
故答案为10．