To write as a polynomial, just multiply every term int he first parentheses by every term in the second parentheses.
It is like distributing:
a2(a-4) - 2a(a-4) + 3(a-4)
a3 - 4a2 - 2a2 + 8a + 3a - 12
Now combine like terms:
a3 - 6a2 + 11a -12 <---- that is your polynomial