Question

This?? refer to comments

Answer 1

This is the definition of the circumcenter of a triangle. http://www.mathopenref.com/trianglecircumcenter.html

It is super-easy to find using geogebra, if that's allowed.

Answer 2

ik what it is can u answer it

Answer 3

Well yes, I can answer it of course. :) I want to help you learn how to answer it on your own though. There are a bunch of ways to find the answer -- does your teacher want you to use any particular method? (Drawing a picture on graph paper, using a program like geogebra, doing the algebra, using a formula, etc.?)