Answer: x = 6 (choice B)
The two chords cut each other in a way that the smaller parts multiply to the same result. Your book would most likely refer to this idea as the "intersecting chords theorem"
One chord is cut into two pieces of 2 and x. The other chord of 4 and 3. The two pieces multiply to the same thing.
So,
2*x = 4*3
2x = 12
2x/2 = 12/2 ... divide both sides by 2
x = 6