Because the 18-gon is symmetrical, you can rotate it by 360/18 or 20 degrees. This is because when you rotate it by 20 degrees(to the left for explanation matters), any angle will now be in the position of its angle to its left.
360 will be divided by 18 which will give you 20. For those of you who had the same question but instead of "18-gon" you had "20-gon" if you divide 360 by 20 your answer will be 18.
