bear in mind the Unit Circle is called so, because its radius is exactly 1, thus just 1 unit, the Unit Circle
anyway [tex]\bf \theta=\cfrac{180s}{\pi r}\qquad
\begin{cases}
\theta=\textit{central angle in degrees}\\
r=radius\\
s=\textit{length of the arc}\\
----------\\
s=4.2\\
r=1
\end{cases}\implies \theta=\cfrac{180\cdot 4.2}{1\cdot \pi }[/tex]
