Answer:
12
Explanation:
By the rules of secants and tangents, ...
... AB² = AC×AD
... 6² = x(x + 9)
... x² +9x -36 = 0 . . . . . subtract 36
... (x +12)(x -3) = 0 . . . . factor
... x = 3 . . . . . . . . . . . . . positive solution
... AD = AC + CD = 3 + 9
... AD = 12