Hello,
To solve this problem, you can use the Pythagorean Theorem.
It is: a² + b² = c²
If you input the 6 for a and the 10 for c, you get:
6² + b² = 10²
Now, you just have to solve the equation.
36 + b² = 100
b² = 64
b = 8
The missing side is 8.
Good luck,
MrEQ