[RyDeR`]

3xІ = 6x
<=> 3xІ - 6x = 0

D = bІ - 4.a.c = (-6)І - 4.3.0 = 36

(-b ± √D) / 2.a

<=> (6 ± 6) / 6

x1 = 2
x2 = 0


That was pretty easy.

EDIT: You already solved it. -.-