Answer:
Completing the square. I mean, you can, for some values of a and b, but it's not going to help much if it is messy. Nope!
Factoring. Yes. It's really easy since there is an obvious common factor.
Square root theorem. No idea what this is. And I don't know how it would help.
Quadratic equation. This works, but is a little long for what you have to do.\