. Mattel has conducted studies to determine the best price to set for their Barbie doll figures. Based on data received through these studies, it was calculated that Mattel could sell 3,000 Barbie dolls at a set price of $17.99. However, it was also determined that if the price per doll was reduced to $9.99, they could sell an additional 2,000 Barbie dolls. Find the linear demand equation (price function, y) as a function of the quantity, x, sold.