Check the picture below. So the sector looks more or less like that one.
we know a full circle has an arc of 2π, so how much is 7/8 of 2π? well, is simply its product.
[tex]\bf ~~\begin{matrix} 2 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~\pi \cdot \cfrac{7}{\underset{4}{~~\begin{matrix} 8 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~}}\implies \cfrac{7\pi }{4}[/tex]
