I assume that you are looking for the equation of the circle? If so, using the distance formula
d = sqrt( (x2-x1)2 + (y2-y1)2 )
the diameter of the circle is sqrt( (2-4)2 + (4+1)2) ) = sqrt(29) = 5.385
To find the midpoint of the circle, you just construct a right triangle with the points as the hypotenuse, then count halfway between the x and y coordinates on each leg.
This gives the mid-point x = 1.5 & y = 3 We know the diameter is 5.385, So the radius is 2.693
Therefore, the equation is (x-1.5)2 + (y-3)2 = 7.25