Hi there!
Before you factor this you must simplify it first.
a(2) - 16a + 64
2a - 16a + 64
- 14a + 64 >> From this point we will be able to factorize it.
-2(7a - 32)
Answer : -2(7a - 32)
I hope that's help and please let me know if you have further questions!