Answer:
A. 7
Step-by-step explanation:
The problem is poorly specified, so technically cannot be answered with a specific number.
If we assume the "horizontal" lines are all parallel, then the one marked 21-x has a length that is the average of the other two:
(17 +11)/2 = 21 -x
14 = 21 -x
x = 21 -14 = 7
The value of x is 7.
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The attachment shows what happens when the lines are not parallel. The range of the midline lengths is from 3 to 14 for the segment lengths shown.
