a^3+2a=-2
3a^6+12a^4-a^3+12a^2-2a-4
=(3a^6+12a^4+12a^2)-(a^3+2a)-4
=3(a^6+4a^4+4a^2)-(a^3+2a)-4
=3(a^3+2a)^2-(a^3+2a)-4
=3(-2)^2-(-2)-4
=10