The circunference is given by the next formula:
[tex]C=\pi\cdot d[/tex]d is the diameter.
Then, you can solve the equation for d:
[tex]d=\frac{C}{\pi}[/tex]Then, for the given circle:
C=47.1ft
[tex]d=\frac{47.1ft}{\pi}\approx15ft[/tex]The diameter is approximately 15ft