Question 1009297
Let the side of the octagon be x.
If you draw the diagram of it, you will notice that
x + x/sqrt(2) + x/sqrt(2) = 18
since the sloping sides are hypotenuses of 45-45-90 right triangles.
Factor out x and get
x(1 + 2/sqrt(2)) = 18
x(1 + sqrt(2)) = 18
x = 18/(1+sqrt(2)) = 7.457 in