The first one: f(x) = 20,000(0.85)^x (make sure you use the carrot thingy when you type these, otherwise you get a linear equation).
If the car is depreciating in value, that means you only have a certain percentage of its value left after a certain amount of time. If you lose 15% of the car's value each time, you're left with 85% of the car's value each time since 15 subtracted from 100 is 85. In order to start this equation, the first time the car depreciates in value, you will be multiplying 20,000 by 0.85 (percentage in decimal form) to determine what the value of the car is after one depreciation. Since this equation is the only one in which 20,000 is multiplied by 0.85, the first equation is correct.
