So with words you won't be able to repeat the letters.
For word possibilities that won't contain one letter assuming the word is 6 letters long that means you have 6 "spaces" to fill. If you want to know how many words there are without using one of the letters, you take the letter away so now you only have 6 letters instead of 7 so it becomes 6*6*6*6*6*6. (you have 6 options to choose from for each space)
If you are saying the word has to have a certain letter, then that space becomes a 1 because there is only one option. But now you have taken that letter away from the seven choices so the rest of the spaces you will only have 6 letter to choose from. So words that contain one of the letters at least once becomes 1*6*6*6*6*6
If you want the same letter twice that means you have a 1 in two of the spaces. 1*1*6*6*6*6