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.