So with your problem, you need to find the leg. By the way, the shortest sides of the triangle and called the legs and the longest is called the hypotenuse. So to find that leg, you need to do c squared - b squared = a squared. So let's work the problem.
Your hypotenuse (C squared) = 13 feet squared
Your leg (B squared) = 8 feet squared
Your other leg (A squared) = x
So now you need to do 13 squared - 8 squared and you should get 105.
Then square root 105 and you should get 10.24. But if the question asks you to round it, it would be 10.2.