18000 represents the price of the new car.
The value of the car in 6 years can be found by substituting 6 for the variable t, as follows:
[tex]V = 18000(1-.12)^6\\V = 18000(0.4644)\\[/tex]
In 6 years, it will be worth around $8359.27.
The equation for the car worth $20000 is this:
[tex]V = 20000(1-.15)^6\\V = 20000(0.3771)\\[/tex]
In 6 years, the other car will be worth $7542.99, so the car worth $18000 will have more value in 6 years.