you are doing fine...

the standard equation of a circle is

from your equation a is h and b is K, it would probably be better to use the standard equation since that is what is commonly used.

first find the center of the circle by doing a simultaneous equations of the 2 perpendicular bisectors

of the lines AB and BC

then you have the center C(h,k)

to find the perpendicular bisector of line AB

squaring both sides yields

next expand

continue with BC then find H,K by simultanuous equation