Would assume the Polygon is an 18 sided regular polygon.
Exterior angle for regular polygon = 360/n (Sum of angles of exterior angles = 360°)
where n = number of sides.
Exterior angle = 360 / 18 = 20
Exterior angle = 20°
Sum of exterior angle + Interior angle = 180° (Both angles are on a straight line, so the sum is 180°)
Interior angle = 180° - 20°
Interior angle = 160°
Hope this explains it.
