Answer:
7.46
Step-by-step explanation:
To find the side of the octagon, we will use the triangles on the corner.
The legs of these triangles are (18 - x)/2 and the hypotenuse is x. So, using the Pythagorean theorem, we can write the following equation

Then, we need to rewrite this expression

Using the quadratic equation, we get that the solution is

x = -43.46 doesn't have sense here, so the length of each side of the octagon is 7.46