David did 9 push-ups in gym class today. David did 6 more push-ups than Zachary. How many push-ups did Zachary and David do altogether?
David did 9 push-ups. 
Zachary did 6 more than David. 
That means Zachary did 9 + 6 = 15 push-ups. 

Now we can add David's 9 push-ups and Zachary's 15 push-ups to find the total number of push-ups: 

9 + 15 = 24

So, David and Zachary did 24 push-ups altogether.