Generally polygons are found drawn on a piece of paper somewhere, perhaps a page in your Geometry book. Sometimes they are drawn on a chalkboard or a whiteboard in your Math classroom. You will know you are looking at a polygon because the geometric figure will have many angles; that is what the word "polygon" means after all. If you really meant, "How do you find the number of sides of a polygon given the sum of the interior angles?" then you should have asked the correct question.
John
Egw to Beta kai to Sigma
My calculator said it, I believe it, that settles it
