Step-by-step answer:
This is a regular heptagon, means it has 7 congruent sides and 7 congruent vertex angles.
To work with polygons, there is a very important piece of information that you must know to solve the majority of related problems.
This is:
sum of exterior angles of polygons = 360 degrees.
If you don't remember the 360 degrees, think of the sum of exterior angles of an equilateral triangle, which is 3*(180-60)=3*120=360! It works!
For a regular heptagon, c = 360/7=51.43 degrees approx.
This means that each vertex angle measures
vertex angle = 180-c
So since 2d+the vertex angle = 360, we have
2d+(180-c)=360
solve for d:
2d=360-(180-c)=180+c
d=(180+c)/2=90+c/2=115.71 degrees. (approx.)