522: Where to start with factoring

You are asked to find the roots and factors of the following polynomial function:
f(x) = x⁴ – 2x³ – 18x² + 6x + 45

By the rational root theorem, possible rational roots are
± 1, 3, 5, 9, 15, or 45.

In order to minimize your effort, you know that you should begin with the possibility that is most likely to be a root. Which one is most likely?