Question 1025762
Hi there,
The brace leaning up against the fence
makes a right angled triangle.
The brace is the hypotenuse
The distance up the fence to
where the brace is nailed is the upright part 
and the distance on the ground from the fence
to the other end of the board is the base.
Base^2 = Hypotenuse^2 - Upright part^2
Base^2 = 8^2 - 6^2
Base^2 = 64 - 36
Base^2 = 28
Base = √28
Base = 5.29 feet (2 decimal places)
Hope this helps :-)