If it is a regular polygon with each interior angle of 120 degrees then it is an hexagon which will have 0.5((6^2)-3*6) equals 9 diagonals
A regular polygon having an interior angle measure of 168o has 30 sides and is called a triacontagon.
Only if the polygon is "regular".
Only when it is a regular polygon that all interior angles are of equal measure
A pentagon (five sided regular polygon) has interior angles of 108°.
If the measure of an interior angle of a regular polygon is 140o, the polygon is a nonagon (sometimes called an ennagon) having 9 sides.
The measure of each interior angle of a fifteen sided regular polygon, a pendedecagon, is 156o.
13 1/3 °
A 13 sided regular polygon
The interior angle of an octdecagon, an 18 sided regular polygon, is 160o.
A regular octadecagon (an eighteen sided polygon) would have nine diagonals.
A regular polygon with 4 sides is a square. The measure of any interior angle of a square is 90 degrees.