where the green point is the center of the circle and red a point
If I draw a line from the center to a point on the circle I will get the radius, so the yellow line is the radius, and I can find it using the formula of distance between two points
where the points are (4,-4) and (1,0)
so, replacing
so the radius is 5
the diameter is 2r so ,replacing
the diameter is 10 units