The key to this problem is that we know, thanks to the question, that this is a right triangle! The side lengths of right triangles can always be found using the Pythagorean Theorem: a2 + b2 = c2 .
This problem gives us side-length a as 8 units, and side-length c as 13 units. We solve by plugging them into the Pythagorean Theorem, like so: 82 + b2 = 132 ; we solve by subtracting 82 from both sides, getting b2 = 132 - 82. Now, all we have to do is solve! The math for b2 works out to 132 - 82 = 169 - 64 = 105 = b2 ; to get b we just take the square root of both sides, and get our final answer as √105!