Derek bought his truck for $30,000. The value has dropped by $900 each year. He determined that the value of his truck (V) can be modeled by: V = 30,000 - 900x where x is the number of years since he bought his truck.
The difference between 30,000 and 4,800 is 25200. So we divide this number by 900 to find out how many years it will take for the truck to depreciate to this value