There's nothing to prove here, this is an equation, and you may solve it.
In order to do so, you must recall that you're allowed to divide both sides of any equation by any non-zero number, and the solution(s) won't change.
In this case, if you divide both sides by 3, you'll get
[tex]3x=6 \iff \dfrac{3x}{3}=\dfrac{6}{3}\iff x=2[/tex]
