So we want to know which number we need to add to 14 to get 17. Lets name that number x. the sum of 14 and x needs to be 17. So we write: 14 + x = 17. Now we transfer number 14 on the other side and change its sign to negative and we get: x = 17 - 14. Finally, we get x=3 so we can write: 14 + 3 = 17.