The sales of a product decline after the end of an advertising campaign, with the sales decay

Question:

The sales of a product decline after the end of an advertising campaign, with the sales decay given by S = 100,000e-0.5x, where S represents the weekly sales and x represents the number of weeks since the end of the campaign.
(a) What will be the sales for the tenth week after the end of the campaign?
(b) During what week after the end of the campaign will sales drop below 400?