In order to get your answer you must remember that if the exponent is negative move to the left and if the exponent is positive you have to move to the right.
If you have built 25 cars, and they are 36 dollars per, then multiply 25 and 36 for the amount of money. This will result with 25*36=900. Since we already have 900 dollars worth, we subtract that from the goal of 1620, leaving us with 1620-900=720. Now, divide this by the price per car for the amount of cars needed to get to this goal. 720/36=20 cars