X=4*Sqrt(2)
By the pythagorean theorem we can see that a^2 + b^2 = c^2. For a 45-45-90 triangle like the one in the question, we see that 2a^2 = c^2 and the answer can be found as follows.
2x^2=8^2
2x^2=64
X^2=32
X=Sqrt(32) (fyi sqrt means square root)
X=4 sqrt(2)