[tex] \bf \textit{arc's length}\\\\ s=\cfrac{\theta \pi r}{180}~~ \begin{cases} r=radius\\ \theta =angle~in\\ \qquad degrees\\[-0.5em] \hrulefill\\ r=75\\ \theta =21 \end{cases}\implies s=\cfrac{(21)(\pi )(75)}{180}\implies s=\cfrac{35\pi }{4} [/tex]
