Answer :

AL2006
Do you remember the formula for the circumference of the circle
when you KNOW the diameter ?

Circumference = (pi) times (diameter)

Divide each side of this equation by (pi) :

Diameter = circumference / pi

Can you handle it from here ?


Cabuji
The fence makes the circumference of your circle.  so, the circumference is 24 ft long. 
Circumference = diameter * pi
so diameter = circumference / pi
d = 24 ft / pi