Circumfrence of a circle formula:
[tex]C=\pi d[/tex]
(C = circumfrence, π = pi, d = diameter)
Let's plug in what we know.
[tex]28=3.14d[/tex]
Divide each side by 3.14 to find d.
[tex]8.917\approx d[/tex]
Round to the nearest foot...
[tex]\boxed{d\approx9}[/tex]