First, make sure that there is a common denominator, or in other words, the denominators, are the same.
You have the numbers 15 and 5 and the lowest common multiple between the two is 15, and in order to make sure the denominators are both the same, you multiply 5 by 3.
Now, whatever you do to the bottom, you make sure you do to the top. In other words, multiply the numerator by 3 as well, and the second fraction is now 3/15. You can now subtract 3 from 13 and the answer is 10/15. You can reduce that to 2/3 by factoring out a 5.
