Formula for sum of interior angles of a regular polygon is given as,
(n-2)×180°
where
n = # of sides of polygon
solve for n,
(n-2)×180° = 1260°
n-2 = 1260/180
n-2 = 7
n = 7 + 2
n = 9
Therefore the polygon has 9 sides.
If the 9 sided polygon is a regular polygon, it will be called a Nonagon.
