Question 657267
This is solved with the Pythagorean theorem which states that the sides of a right triangle a, b, and c fit the equation:
a^2+b^2=c^2 where c is the hypotenuse which is the longest side.

So:

6^2+x^2=10^2
36+x^2=100
36+x^2-36=100-36
36+x^2-36=100-36
x^2+0=64
x^2=64
x=sqrt(64)
x=8