Let P(x,y) be an arbitrary point on circle M.  The right triangle with
legs x and y, hypotenuse 5, and vertex P(x,y) can be drawn anywhere on the
circle.  So the equation that holds for all points on the circle is the
Pythagorean theorem, using x and y for the legs and 5 for the hypotenuse.

x2 + y2 = 52

or

x2 + y2 = 25

Edwin