He then went to see the oranges being harvested. He found out that they harvest w sacks per day and discard x of them. How many sacks of oranges will they have after y days of harvest?
Let's denote the number of sacks of oranges by n.

n = w * y - x * y

After y days of harvest they will have w * y sacks of oranges and they will discard x * y sacks of oranges.

Therefore, the answer (symbolic expression) is: 

w * y - x * y