Here is my answer:
If one leg is x.
Then the other legs need to be longer than 7, so it is (x + 7)
x² + (x +7)² = 13²
x² + (x +7)² = 169
So I believe that is the equation.
From the Pythagorean triple of 5, 12, 13, one should and would know that if the equation above is solved, x = 5.