Since the triangle is a right-angle triangle,
Using Pythagoras Theorem,
x^2 + x^2 = 8^2
2(x^2) = 64
x^2 = 32
x = √32 or - (√32)
However, since x is a positive value,
x = √32
Sorry, I don't have a calculator with me right now, but if you want the exact value of x expressed to 3 sig. fig., you can type √32 into a calculator and find out