Question 30933
I assume your triangle looks like this and has a right angle: 


10/|
 /  |||x
-----
 8

If I assumed incorrectly, then please provide more information otherwise to solve for x, you will need to use the pythagorean theorem where a^2 + b^2 = c^2 and c is the hypotenuse.  
In this case, our "a" is 8, "b" is x, and "c" is 10.  Substitute and solve for x: 
8^2 + x^2 = 10^2
64 + x^2 = 100
x^2 = 100 - 64
x^2 = 36
x = 6