What is the greatest prime factor of 4^17 - 2^28?

1) 2
2) 3
3) 5
4) 7
5) 11

This doesn't seem that difficult, but after I reduce it to 4^17 - 4^14 or 2^34 - 2^28, I don't see what else I can do. I'm sure I'm missing something quite easy.