Answer:
Denote the O, A, B, C, D as shown in attached image:
O is center of circle.
OA = OC = OD = radius of circle
=> triangle OAD is isosceles
=> BA = BD = 12.8
Applying Pythagorean theorem:
OB = sqrt(OA^2 - BA^2) = sqrt(15.1^2 - 12.8^2) = 8.0
=> x = OC - OB = 15.1 - 8.0 = 7.1
Hope this helps!
:)
