As a first step we can notice that in three hours at a rate of 52 boxes an hour, they produce:

If they have 404 boxes in stock, they must have had:

when the store opened.
Then, the expression that represents the situtation would be:
Let b the number of boxes.
Let h number of hours since they opened.

Now, to calculate how many boxes they will have at the end of an 8 hour shift, we just have to susbtitute h=8.

At the end of the shift, they will have 664 boxes.