Work backwards with pythagoras' theorem:
c^2=a^2+b^2
Here, a and b both equal to x. Therefore, you can write this as 8^2=2x^2. So 64=2x^2. Sqrt on both sides to give you 8=2x, hence x=4. However I must warn you that this working out may have a flaw, as I found that x actually equals to 5.66
