You have to start with the left triangle and find the side lengths. Since it is a 45-45-90, both sides are the same. Rule for this is hyp= sqrt2 * leg
Since hyp = 6sqrt2, the leg is 6.
The leg is also the hypotenuse of the triangle on the right side.
x is the side across from the hypotenuse and it is the short leg so x = (1/2)6
x = 3