Answer:
4
Step-by-step explanation:
By intersecting chords theorem:
[tex]12x = 3(x + 12) \\ \\ 12x = 3x + 36 \\ \\ 12x - 3x = 36 \\ \\ 9x = 36 \\ \\ x = \frac{36}{9} \\ \\ x = 4[/tex]
