From Excel, the linear regression model that best fits the data (so, an equation that gives the value of the car, y, as a function of its age, x) is:

y = -1483.27x + 20,575.93

The slope of the line (-1483.27) is negative, indicating that value of the car decreases by $1483.27 for every 1 year increase in age. The y-intercept of the line is 20,575.93, which tells us that a new car (age of 0) is expected to have a value of $20,575.93.

You can find this equation by putting the data into Excel, then highlighting the two columns of data and inserting a scatterplot into the spreadsheet. In editing the scatterplot, you can select the option to insert a linear regression line with the equation on the plot - you will get a picture that looks like the attached document.