Leon verified that the side lengths 21, 28, 35 form a Pythagoreom triple using this procedure

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