Nell collects baseball cards. She had 42 cards. She gave 9 cards to John and yet some more to Jeff. Now she has 28 cards left. How many cards did Nell give to Jeff?
Nell has 42 cards. 
She gave 9 cards to John. 
That means she has 42-9=33 cards left. 
She gave some more cards to Jeff. 
That means she has 33-x=28 cards left, where x is the number of cards she gave to Jeff. 

We can solve this equation for x: 

33-x=28
33-28=x
x=5

Nell gave 5 cards to Jeff.