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 are a total of x campers that went rowing that day. w campers went rowing in the afternoon, which means that the other campers must have gone rowing in the morning. 

This means that the number of campers that went rowing in the morning is x - w.

Therefore, the answer (symbolic expression) is: 

x - w