This involves solving several systems of equations,
either by substitution or elimination:




First, find the vertices of the triangle by solving
the three systems of equations by either substitution
or elimination.

    

The solutions are (0,0), (7,7) and (10,-5)

Those are points on the circle so they will satisfy
the equation:



Substituting the three vertices gives this system:

 

which simplifies to:



And since F=0 we only have two equations:



Dividing the first equation thru by 7 and the 2nd one by 5,



or



solving that systen gives D=-13, E=-1

So the general equation 

 bcomes



Edwin