From the figure, the chord (12) is drawn with a line (from the center of the circle) perpendicular to it. It can be said that the chord 12 is bisected by the 7 long line from the center which gives us with 6 each of the two parts of the chord. Since they are forming (the 6 and 7) a right angled triangle, we can then use Pythagorean theorem to find X, which can be seen as the radius at the same time hypotenuse of the right angled triangle.
