If the two given sides are the legs of the triangle, you recognize it as a 3-4-5 triangle scaled by a factor of 2. The hypotenuse is 10 units long.
If the length 8 is the hypotenuse of the triangle, the short leg is
.. √(8^2 -6^2) = √(64 -36) = 2√7 ≈ 5.292 . . . . . units long
The possible lengths of the third side are 2√7 and 10 units.