Please help me.
The center of the circle is going to be at the half-way point of the diameter (or equal to the radius of the circle).
If you draw yourself a quick sketch of the two points you will see that the center point is equal to half the horizontal distance between the two points and half the vertical distance between the two points.
To work out these differences, add up the horizontal co-ordinates and half them (X1 + X2) / 2
and do the same to the vertical co-ordinates (Y1 + Y2) / 2
This will leave you with the co-ordinates of the center point of the circle.