Answer:
13
Explanation:
You want to know the length of segment x in the similar triangle figure.
Proportional
The proportional relationship between the short side and the long side of each of the triangles tells you they are similar, and that the relationship extends to the unknown side.
11/(11+22) = 12/(12+24) = x/39 = 1/3
x = 39(1/3) = 13
The value of x in this figure is 13.
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Additional comment
Clockwise from left, the triangle side lengths form an arithmetic progression:
33 : 36 : 39 = 11 : 12 : 13 = 11 : 12 : x ⇒ x = 13