leave answer in terms of pi or as a fraction.

We are asked to find the arc length of a segment of a circle, to do that let's remember the formula for the arclength of a circle:
[tex]s=r\theta[/tex]Where the greek letter theta represents the angle in radians. Since we are given the angle in degrees we need to transform it using the following relationship:
[tex]\theta_{radians}=\frac{\theta_{degrees}\pi}{180}[/tex]Replacing the value of theta we get:
[tex]\theta_{radians}=\frac{315\pi}{180}[/tex]Simplifying we get:
[tex]\theta_{radians}=\frac{7\pi}{4}[/tex]Replacing in the formula for the arc length:
[tex]s=(12)(\frac{7\pi}{4})[/tex]simplifying:
[tex]s=21\pi[/tex]