We start with a value of 17000 in 2006. For reasons that will soon be clear, let's write it as
[tex]f(0)=17000=17000\cdot (0.79)^0[/tex]
One year later, the resale value has decreased by 21%, which implies that the resale value is 79% of the previous one: the resale value in 2007 is
[tex]f(1)=f(0)\cdot 0.79 = 17000\cdot (0.79)^1[/tex]
Move again one year forward: the resale price in 2008 is 79% of the resale price in 2007:
[tex]f(2)=f(1)\cdot 0.79 = (17000\cdot 0.79)\cdot 0.79 = 17000\cdot (0.79)^2[/tex]
So, as you can see, after t years from 2006, the resale value will be multiplied by 0.79 t times, leading to the function
[tex]f(t) = 17000\cdot(0.79)^t[/tex]
