interior angle of a regular 18-gon. It is easier to calculate the exterior angle of a regular polygon of n-sides (n-gon) by the relation exterior angle = 360/n For a 18-gon, n=18, so exterior angle = 360/18=20 ° The value of each interior angle is therefore the supplement, or Interior angle = 180-20=160 degrees.
Naming of a 9-gon A polygon with 9 vertices is called a nonagon (in English) or enneagon (French ennéagone, but the English version is sometimes used) You had a good start with the correct answer.
Exterior angle of a 15-gon The exterior angle of a 15-gon can be calculated using the relation given in the first paragraph, namely Exterior angle = 360/15=24 degrees