You can factor out a -4 since that can be easily divided into 12 and
you can take out an a^3 since that can be easily divided into a^5 and
a^4...so in the end you can end up factoring out an entire -4a^3 out of
each of the three parts of the polynomial:
-12a^5 / -4a^3 = 3a^2

12a^4 / -4a^3 = -3a

-4a^3 / -4a^3 = 1

So in final factored form your polynomial looks like: