First you must know that the interior angles of a triangle add up to 180 degrees. Once you recognize that, you can add 2x+3x+4x=180. Then you just solve for x.
2x+3x+4x=180 becomes 9x=180.
Then you divide both sides by 9 and x=20 is your answer.
For future reference, if you need to solve for the angles after finding x, you would just plug in the 20, giving you three angles 40, 60, and 80.
