Since you said "hypotenuse" and "legs", I assume you're talking about a right
triangle, although you didn't say that, and the legs are the two equal sides.
Since it's a right triangle, A² + B² = H².
In this particular one, A = B , so you can say that
A² + A² = H²
2A² = H²
H = square root of (2A²) = A√2
Since you know 'H' and you're looking for 'A', divide each side by √2 :
A = H / √2 .