#### Answer

$r=\dfrac{C}{2\pi}$

#### Work Step by Step

Using the properties of equality, in terms of $
r
,$ the given formula, $
C=2\pi r
,$ is equivalent to
\begin{array}{l}\require{cancel}
\dfrac{C}{2\pi}=\dfrac{2\pi r}{2\pi}
\\\\
r=\dfrac{C}{2\pi}
.\end{array}