Some campers went rowing in the morning while w campers went rowing in the afternoon. If a total of x campers went rowing that day How many campers went rowing in the morning?
There were some campers rowing in the morning. That means that not all of the campers went rowing in the morning. So, we can write:

x campers went rowing that day 
- w campers went rowing in the afternoon 
= some campers went rowing in the morning

We don't know how many campers went rowing in the morning, so we'll call that number y. So we have:

x campers went rowing that day
- w campers went rowing in the afternoon
= y campers went rowing in the morning

We can solve this equation for y:

y = x - w