We can use the Pythagorean theorem. This is a²+b²=c²
So let's break this down. C is the hypotenuse. That is 13. We will use b for 12 you can also use a it doesn't really matter.
So this turns into a²+(12)²=(13)² Which equals a²+144=169 Subtract 144 You get a²=25 Now to get rid of the squared, you square root the a and the 25 so it becomes just an a. This brings us to a=√25 Which means a=5