Find the length of a circle in geometry?

Exactly the same way you find any missing item in any other subject:
You take the information you’re given, and you collect everything you know about
relationships between the information you have and the item you have to find.
Finally, you use the relationships and the given information to find the value of the
missing item.

The process you’ll use to find the circumference of a circle will depend on what
information you DO have about the circle.

The length of the circle is the same thing as its circumference
circumference= 2×π×radius²