First we know that a^2+b^2=c^2. We only know a and c so we are trying to find b. You square both numbers and subtract.
13^2 - 8^2=105. Then what you do is square root. √105=10.24695077
Then depending on how many significant figures or decimal places it says you round
(By the way its really hard to understand the numbers ,so the numbers i put might be wrong im not sure. Are they 13 and 8?)