This problem uses the Angle Bisectir Theorem, which states that (in this problem):
The side measuring 12 over the side measuring 8 is equal to the side measuring x over the side measuring 6.
Basically, we can write this as a proportion:
12/8 = x/6
Cross multiply:
8x = 72
Divide both sides by 8:
x = 9
The answer is (D), x = 9.