On an x-y coordinate plane, suppose x represents the year that a car was sold and y represents the sale price of the car. Rochelle bought a used car for $14,000 in 2004. She sold the car for $6,200 in 2010. Use the slope of the line connecting the points representing the two car sales to calculate the depreciation of the car's value per year over the time Rochelle owned it. (Enter the depreciation as a positive value.)