Answer:
x = 7.5
Explanation:
If two scants are drawn from a point outside the circle then the product of one secant and its external segment is equal to the product of other secant and its external segment.
By applying this property in the picture attached,
OB × OA = OC × OD
(6 + 3)6 = (12 - x)12
54 = 144 - 12x
12x = 144 - 54
12x = 90
x = 7.5