The Pythagorean Theorem is: 

So, for your problem, the hypotenuse is 13 and another side length is 5.
It doesn't matter if you put 5 in the equation as a or b. 
The equation would be: 

To solve you would do 5*5=25 and 13*13= 169.
You would end up with: 

Subtract 25 from 169 to get 144. In order to get b by itself, you must square root both sides. The other side length is 12.
Then multiply 12 by 5 to get 60
Then divide 60 by 2 to get 30. 
THE ANSWER IS 30.