This isn't exactly about an assignment question but rather about the topic, "Arc length". I seem to may grasp it, but I'm stuck on how to get the answer. How does, like, the fraction become the answer

This isn't exactly about an assignment question but rather about the topic, "Arc length". I seem to may grasp it, but I'm stuck on how to get the answer. How does, like, the fraction become the answer? Nothing I calculate gets me the right thing, and I'm having trouble finding how its even done or something that says.