Hello there! How are you doing? I am doing fine! I'll answer the question now.
So basically when wanting to find the circumference of a circle you simply use the formula, C = 2 times PI times R. PI is the infamous number known for 3.1415912654... and the R represents the radius. You would multiply 2 times PI and then after getting that you would basically multiply it by the radius. And then that would be the circumference and be the answer to any problem you face like this. Know that diameter is 2 times the radius and that all.
Hope this helped! Your welcome! I can help with other history stuff too! And byee! :)