+1678 The number of diagonals in a certain regular polygon is equal to $2$ times the number of sides. How many sides does this polygon have?
+57 So basically is there are x sides, there will be x*(x-1)/2 - x diagonals, and if 2x = x*(x-1)/2 - x, then x must equal to 7, septagon. Go on, try it!
