You can't add 3x. Your goal to solve for x is to isolate it. To do so, move the term with a variable to one side and the constant to the other. To do so, subtract both sides by x then subtract both sides by 2 and simplify from there to get x completely on it's own.

What you do on one side of an equation, you must do on teh other side to keep the equation the as having teh same value. By doing so, you alter the form of teh equation while keeping teh meaning.