Using Pythagorean theorem:
c² = a² + b²
where a and b are the length of the legs, and c is the length of the hypotenuse.
Given that the triangle is a 45-45-90 triangle, then both legs have the same length, that is, a = b. Then,
c² = a² + a²
c² = 2a²
√c² = √(2a²)
c = √2a
The length of the hypotenuse is found, multiplying the leg's length by √2