The formula for the area of a triangle is
a = 1/2 × base × height
Since we are breaking apart the octagon into 8 equal triangles, the height of each triangle will be the distance from the midpoint of the octagon to the midpoint of one of the sides which is given as 10.86 cm. The base is given as 9 cm...
a = 1/2 base × height
a= 1/2 (9 × 10.86)
a = 1/2 × 97.74
a = 48.87 cm^2
Now don't forget... this is the area of one of the triangles and we have eight of them, so...
48.87 × 8 =
390.96 cm^2 (Total area)
