If I have an octagon that is 12 feet across (side to opposite side) how can I find the length of the sides?

THank you

I assume you have a regular octagon so you can think of it as a 12 ft by 12 ft square with the corners cut off. Suppose that each side has length S.

In the diagram the corner triangle, ABC, has |CA| = |BC| and thus using the theorem of Pythagoras

|CA|² + |BC|² = S²
2 |CA|² = S²
|CA| = S/

Since the octagon is 12 feet across,

S + 2(S/) = 12

S = 4.97 feet