PLEASE HELP ME IF YOU CAN! please tell me how you got the answer thank you!

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)