The equation of the circle having (4,2) and (2,−12) as the end points of its diameter is
x2+y2−6x+10y−16=0
Equation of a circle when the end points (x1,y1) and (x2,y2) of its diameter are given, is given by,
(x−x1)(x−x2)+(y−y1)(y−y2)=0
So, when (4,2) and (2,−12) are the end points of the diameter, equation of the circle would become:
(x−4)(x−2)+(y−2)(y+12)=0
x2−2x−4x+8+y2+12y−2y−24=0
x2+y2−6x+10y−16=0