I'm in a Systems & Signals class and I need to find the stability of a system.

I know that if the roots to the characteristic equation of the system are equal or less than 1, the system is stable. My problem is it's been a while since I've had an Algebra course and I haven't had to factor a polynomial like this in a long time.

The equation:

a^4 - (2/3)a - 1/3

How would I go about finding the roots to that polynomial?