There is a common factor of 3x.
I believe that the first to approach such a problem will to to group the like terms all to one side. After grouping the terms, you can then start to pulling out the common factors that appeared throughout the expression.
3x^2 = 6x
3x^2 - 6x = 0
3x (x - 2) = 0
Thus leaving your final answer in the parentheses.