Answer: 4√2
Explanation:
The triangle is a special right triangle. Using this, you know that the legs of the triangle are 1 and 1 as the 45, 45, 90 triangle has the sides of 1, 1 and √2. That being said,
4 x 1 = 4 - This is one of your sides
4 x 1 = 4 - This is your other side.
4 x √2 = 4√2 = This your your hypotenuse or your x.
My teacher played me this song to help me remember 45, 45, 90 and 30, 60, 90 triangles. Look up "45 45 90 30 60 90 song" and look for the channel, "Scalar Learning"
I hope this answered your problem.