Hello! We will be using the Pythagorean theorum, where a² + b² = c². C is the clanted part of the triangle, and 13² is 169. a = 12, and we are looking for the value of b. 12² is 144. Let's subtract to find b. 169 - 144 is 25. Now, let's find the square root of 25 to find the length. √25 is 5. There. b = 5. The side length of side b is 5. The correct answer is C: 5.