It is
+ = ,
which is the same as
+ = 68.
Notice that the right side is the square of the radius,
and the radius in this case is the distance of the given point from the origin of the coordinate system.
Solved.
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By the way, the circle is shown INCORRECTLY in the post by @MathLover1.
Would it be plotted correctly, the circle must go through the point (4,0), too.