we know that
The nine radii of a regular Nonagon divides into 9 congruent isosceles triangles
therefore
[the area of a regular nonagon]=9*[area of isosceles triangle]
[area of isosceles triangle]=b*h/2------> 15*20.6/2----> 154.5 cm²
so
[the area of a regular nonagon]=9*[154.5]------> 1390.5 cm²
the answer is
1390.5 cm²
