a³+2a=-2
3（a的六次方）+12a⁴-a³+12a²-2a-4
=3（a的六次方）+12a⁴-(a³+2a)+12a²-4
=3（a的六次方）+12a⁴+12a²+2-4
=3(a³+2a)²-2
=3×4-2
=10