You have that the distance of the segment CA is the sum of the segment CB and BA, as you can notice in the given figure.
In order to solve for x, you simply take into account the previous situation, segements CB and BA are equal to CA. You put this in an algebraic way, and solve for x. You proceed as follow:
x - 8 + 11 = 2x - 8 add 8 both sides
x + 11 = 2x subtract x both sides
11 = x
Hence, x is equal to 11