as you can see, the central angle for the arcAB is 270°, whilst the radius is 4 units long.
[tex]\bf \textit{arc's length}\\\\
s=\cfrac{\theta \pi r}{180}~~
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad degrees\\
------\\
\theta =270\\
r=4
\end{cases}\implies s=\cfrac{(270)(\pi )(4)}{180}\implies s=6\pi [/tex]
