We have the following general rule to find the length of an arc:
![Arc=r\cdot(\alpha\cdot(2\pi))/(360)](https://img.qammunity.org/2023/formulas/mathematics/college/2svf8bgal1qemcaf5n1hrx49ojvuopqswl.png)
where r is the radius and alpha is the measure of the central angle in degrees.
In this case, we have that r = 8 and alpha equals 270 degrees, since that is the measure of the angle that completes the full circumference, then:
![\begin{gathered} Arc=8\cdot((270\cdot2\pi)/(360))=8\cdot((540)/(360)\pi)=8((3)/(2)\pi)=(24)/(2)\pi=12\pi \\ \Rightarrow Arc=12\pi \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/coxszxkokriips5v5dy3whb1jggyz0vjr3.png)
therefore, the length of the gray arc is 12 pi