David did w push-ups in gym class today. David did x more push-ups than Zachary. How many push-ups did Zachary and David do altogether?
First, we need to know how many push-ups Zachary did. We know that David did more push-ups than Zachary, so we'll start by subtracting x from w. 

w - x = y

That means that y is the number of push-ups Zachary did. 

Now we need to add y and w together to get the total number of push-ups done by both Zachary and David. 

y + w = total number of push-ups

We can plug in what we know for y and w to solve for the total number of push-ups. 

w - x + w = total number of push-ups

w - x + w = 2w - x

2w - x = total number of push-ups