Answer: 5
Explanation:
For this question you will have to use pythagoras' theorem.
The formula for the short side is a²=c²-b² and when you put the numbers from this question into it you get a²=13²-12²
13²=169
12²=144
a²=169-144
a²=25
Then as you want to find a on it's own you have to square root 25 which is 5.