Hello, and welcome to FMH!

The arc-length \(s\), radius \(r\) and central angle \(\theta\) are related as follows:

\(\displaystyle s=r\theta\tag{1}\)

The circumference \(C\) and radius \(r\) of a circle are related as follows:

\(\displaystyle C=2\pi r\tag{2}\)

What I would do is solve (1) for \(\theta\), and then solve (2) for \(r\), and substitute for \(r\) into the equation resulting from solving (1) for \(\theta\). Then you will have \(\theta\) as a function of \(s\) and \(C\), both of which you've been given. What do you get?