3sqrt(2)
You can either use Pythagorean theorem or special triangle proportions to find the side that is parallel to z. Since in a 45-45-90 degree triangle, the hypotenuse is the leg times the square root of 2, and the hypotenuse is 3, we know that the leg is 3/sqrt(2) which is equal to 3 sqrt(2) / 2. Since z is twice this length, we know that z is equal to 2 x 3sqrt(2)/2 = 3sqrt(2)
