The correct answer is 9.
Because they share a common angle at the top, we know that we can create a proportion using similar sides.
[tex] \frac{Outside LEFT}{Bottom LEFT} = \frac{Outside RIGHT}{Bottom RIGHT} [/tex]
[tex] \frac{12}{8} = \frac{x}{6} [/tex]
Now we can cross multiply to solve.
12*6 = 8*x
72 = 8x
9 = x
