3 because a polygon has 3 or more sides
Lets assume center point of circle as O
OD is the radius of circle . so OD is also 7
All the sides of the triangle are equal . so angle at the center O is 60 degree.
radius = 7
Length of arc CD = 
= 
= 
= 
¿Qué no entiendes para que pueda ayudarte?