Ineed with pythagorean theorem not really good with this

The pythagorean theorem is a^2+b^2=c^2. Since you have a and c you must solve for b. you would write x^2+8^2=13^2. Which is x^2+64=169. Subtract 169-64, which is 105. you would then write radical 105, which is approximately 10.2. x=10.2

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.