If i am only given the hypotenuse on an isosceles triange (15) then how would i find the legs?

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 .