Answer:
7.2
Explanation:
(We presume a typo in the problem statement. We expect CD = 9. As it stands, the only thing you can say is that x is between 4 and 13.)
By the rules of secants and tangents, ...
... AB² = AC×CD
... x² = 4×(4+9) = 52
... x = √52 ≈ 7.2