pull out the common factor of 3, which leaves you:
3(x^2 - 5x + 6) = 0 ----> 3(x-3)(x-2) = 0
x= 3 and x=2
I'm afraid this is not what the OP asked for, though she/he seems to be unaware of it. What you did here is to factor the quadratic, which is very useful to calculate its roots, for example, but it's not completing the square.