Factorising polynomials resulting in surds I am trying to factorise $x^2-18x+60$.  Wolfram Alpha tells me this factorises to $(x-\sqrt21-9)(x+\sqrt21-9)$, but what technique should I be using to find this myself?

HINT:
$$x^2-18x+60=x^2-2\cdot x\cdot9+9^2+60-9^2=(x-9)^2-(\sqrt{21})^2$$
Now use $a^2-b^2=(a+b)(a-b)$
