I can suggest two approaches to solve this problem. The first is very visual.
Find 14 objects. Marbles, jelly beans, pencils anything. Put them in a pile. Now separate the objects into two piles. Count the number of items in each pile. The sum of these numbers is 14. What is the difference between the two numbers? Can you arrange the objects into two piles so that the difference of the numbers in the two piles is 2?
What number, added to itself gives 14? In other words what is half of 14? Now you have two numbers, actually two copies of the same number, with a sum of 14 but their difference is zero. Increase one of the numbers by one and now the sum is 15. If you decrease the other number by one the sum of the two numbers is now 14. What is their difference?
I hope this helps,