The segment that is x units long is the midsegment of the largest triangle. The midsegment is half as long as the line parallel to the midsegment. It is half as long as the 30 unit long segment.
Note how the vertical sides (marked with the double tickmarks) are congruent to one another. The smaller pieces are both the same length, so they combine to something twice as big. Go in reverse: one segment with a pair of double tickmarks is exactly half as long as the entire vertical side. The same can be said about the horizontal sides as well.
