We have the following equation:
[tex]C=2\pi r[/tex]We want to rewrite it in a way 'C' is a function of 'r'. We can accomplish that, if we divide both sides by 2pi.
[tex]2\pi r=C\Rightarrow r=\frac{C}{2\pi}[/tex]From this, we have our result.
[tex]r=\frac{C}{2\pi}[/tex]