'x' is a radius of the circle.
What happens when you connect the center of the circle
to one end of the chord ? The line that you drew is another
radius, so it has the same length as 'x'.
You also create a right triangle. One leg is 6, and the other leg is 12.
The radius that you drew is the hypotenuse of the triangle, so its
length is
√(6² + 12²) = √(36 + 144) = √180 = 13.4164... (rounded)
