The sum of 14 and a number is equal to 17.

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.