Geometry

The answer is C. 15.

 

1. Observe the congruence of line - segments in the diagram:

In the geometric diagram, line - segments with the same number of tick marks are congruent. Here,  and  both have one tick mark, so .

 

2. Use the property of congruent line - segments:

The property of congruent line - segments states that if two line - segments are congruent, then their lengths are equal. That is, if , then .

We are given that

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