Because every triangle's angles add up to 180 degrees, you know 3x+x+20=180. Since 3x and x are like terms, you can add them to get 4x. 20 and 180 are also like terms. However, to move your 20 to the side of 180, you need to subtract it. So you have 4x=160. To get x by itself, you divide 4x and 160 by 4. 4x/4 is x, and 160/4 is 40. So your answer is x=40.
