Step 2: Go find the correct measurements of your octagon. A regular octagon with an apothem of 6 units has a side length of approximately 4.83 units. An octagon with an apothem of 6 units and one of the sides that measures 5 units is not regular.
Once you know the correct values use where is the span of the octagon which is twice the apothem, and is the measure of one side.
John
My calculator said it, I believe it, that settles it