Answer:
c. 12
Explanation:
Deduct 11 from 21 to get 10, then split it in half, to get 5 as your two bases for the right triangles. Now, according to the Pythagorean Theorem, a² + b² = c², in this case, since the hypotenuse, c, is already defined, use Subtraction to find the other leg:

25 + b² = 169
-25 - 25
______________
√b² = √144
b = 12
When we are talking about lengths, we want the NON-NEGATIVE root.
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