Answer:

Step-by-step explanation:

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Answer:
16.5 inches
Step-by-step explanation:
Use the Pythagorean Theorem
a² + b² = c²
a² + 13² = 21²
a² + 169 = 441
Subtract 169 from both sides
a² = 272
Take the square root of both sides
a = 16.4924225025
Rounded
a = 16.5 inches
Probably a plane unless there's more to the question