3x² - 21
Factor 3 from both terms:
3 (x² - 7)
To me, that's as far as you should need to go. But if you want to get
completely carried away, you could go one step further, since you have
the difference of two squares:
3 (x + √7) (x - √7)
Of course, there's no end now, because the last binomial could be
considered another difference of two squares, so you'd have to
factor that too:
3 (x + √7) (√x + ⁴√7) (√x - ⁴√7)
but to me, this would be nonsense.