[tex]\bf \textit{circumference of a circle}\\\\
C=2\pi r\quad
\begin{cases}
r=radius\\
-----\\
C=28.26
\end{cases}\implies 28.26=2\pi r\implies \cfrac{28.26}{2\pi }=r
\\\\\\
\cfrac{14.13}{\pi } =r[/tex]
now, that's just the radius, recall the radius is half the diameter, so the diameter is twice as much as the radius, so the diameter is two times that much.