There's no such shape as a 'plolygon' but a circle has 360 degrees
((180 x 6) - 360)/6 ie 120o. or 180 - (360/6)
It depends on how many sides that it has and whether of not it is regular (all the lines of the polygon are of equal length if it is a regular polygon). For regular polygons, the number of symmetry lines is the number of sides if number of sides is an odd number. Otherwise, the number of symmetry lines is double the number of sides. A square has 4 sides and 8 symmetry lines; a triangle has 3 sides and 3 symmetry lines.