You don't need to do all that calculating the other tutor did.

First we plot the points:

 

Then we draw the triangle:



That is an isosceles right triangle, so the midpoint of
its hypotenuse is equidistant from the three vertices.

So the midpoint is found by averaging the coordinates
of the endpoints of the hypotenuse:




 
If you like you can draw a circle with that point as the center
and the three given points will be equidistance from the center:





Edwin