w campers went rowing in the morning. Some more campers went rowing in the afternoon. If a total of x campers went rowing that day How many campers went rowing in the afternoon?
Let's denote the number of campers who went rowing in the morning by m.

m = w

Let's denote the number of campers who went rowing in the afternoon by n.

n = x - w

Therefore, the answer (symbolic expression) is: 

x - w