Answer: The missing side is 12.
Step-by-step explanation: Following the equation a² + b² = c², b=5 and c=13, which leaves A. 5² is 25, and 13² is 169. Subtract 25 from 169 and your left with 144, of which the square root is 12. so A=12.
Inserting these numbers into a² + b² = c² would make 5² + 12² = 13², which would then make 25 + 144 = 169, which would make 169=169. Sorry if the process doesn't make sense, but the answer is 12.