12a^4-3a[a^2-4a(a^3-2a^2+3a)]
=12a^4-3a(a^2-4a^4+8a^3-12a^2)
=12a^4-3a(-4a^4+8a^3-11a^2)
=12a^4+12a^5-24a^4+33a^3
=12a^5-12a^4+33a^3