Answer:
7.5
Step-by-step explanation:
By intersecting secants theorem:
[tex]12(12 - x) = 6(6 + 3) \\ \\ 12(12 - x) = 54 \\ \\ 144 - 12x = 54 \\ \\ 144 - 54 = 12x \\ \\ 90 = 12x \\ \\ x = \frac{90}{12} \\ \\ x = 7.5[/tex]
