Can someone explain to me why is it increase of $1,000?

5.Data concerning Peyton Corporation's single product appear below: The company is currently selling 6,000 units per month. Fixed costs are $424,000 per month. The marketing manager believes that a $7,000 increase in the monthly advertising budget would result in a 100 unit increase in monthly sales. What should be the overall effect on the company's monthly net operating income of this change? Decrease of $7,000 Increase of $1,000 Increase of $8,000 Decrease of $1,000