I need to know the measurement of the hypotenuse, but I can help you understand the concept!
A 45° - 45° - 90° triangle has special properties because you can already know the ratio of the sides. The two legs (smaller sides) both measure x, and the hypotenuse is x√2. So if the smaller sides are both 6, the hypotenuse measures 6√2.
If you are given the length of the hypotenuse and you need to find the length of the other sides, use this ratio. 1/ √2 = x / length of the hypotenuse. This is using the ratio of a 45-45-90 triangle to help you find your missing side length. Then you can cross multiply and find that √2x = length of hypotenuse, or x = length of hypotenuse / √2
Hopefully this helps!