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?
First, we need to find out how many sacks of oranges they will have after one day of harvest. We know that they harvest w sacks per day and discard x of them. This means that they will have w - x sacks of oranges after one day of harvest. 

Now, we need to find out how many sacks of oranges they will have after y days of harvest. To do this, we need to multiply the number of sacks they have after one day of harvest by the number of days of harvest. We know that they will have w - x sacks of oranges after one day of harvest, so they will have (w - x)*y sacks of oranges after y days of harvest. 

This means that the answer (symbolic expression) is: 

(w - x)*y