we know that
The area of the circle is equal to
[tex]A=\pi r^{2}[/tex]
where
r is the radius of the circle
In this problem we have
[tex]A=64\pi \ ft^{2}[/tex]
substitute in the formula and solve for r
[tex]64\pi=\pi r^{2}[/tex]
Simplify
[tex]64=r^{2}[/tex]
Square root both sides
[tex]r=8\ ft[/tex]
therefore
the answer is
The value of r is [tex]8\ ft[/tex]