By expansion, I mean multiplying out
and the third part of that equality,
as you need to go through those steps, to find the x and y co-ordinates of the circumcentre.
The other way is to write the equations of two of the three perpendicular bisectors of the sides of the triangle.
Then you find the point of intersection of these 2 lines.
you need to work with the geometric diagram of the circumcentre that i uploaded or some other suitable one.
You need to do this carefully step by step, as you are learning this.
So, just reply saying which method you want to continue with first.