Answer: x = 9
This is because the two chords are the same length, so they are the same distance away from the center.
We can prove this by drawing the following red segments as shown below. These segments are radii of the circle. After forming the two right triangles, note how we can use the hypotenuse leg (HL) theorem to prove the triangles congruent. So this leads to x = 9 because the corresponding pieces of the triangles must be the same.