Verified Answer :

nobrains

21, 28, and 35 are multiples of 7. So you can divide each number by 7 (in other words scaling the triangle down by a scale factor of 7).
It becomes 3,4,5, which is the base Pythagorean triple.
3^2 + 4^2 = 5^2
9 + 16 = 25

Answer:

The correct answer is  25

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