You have the following data points (t,p) where t is the year and p is the price:

(0, 26000)

(1, 24800)

The equation for a line is y = mx + b where y is the dependent variable, x is the independent variable, m is the slope and b is the dependent variable's intercept. In this case, then, we have p = mt + b.

m = (p2 - p1)/(t2 - t1) = (24800 - 26000)/(1 - 0) = -1200. (In units of $ per year.)

So p = -1200t + b.

Now, we know that when t = 0, p = 26000, so

26000 = -1200*0 + b => b = 26000.

Thus p = -1200t + 26000 is the price as a function of time.

To find when the car is worth $500, we set p = 500 and solve for t:

500 = -1200t + 26000

I get t = 21.25 years.

-Dan