C.
The total cost of the drum set is $360 and she already has $80 saved. In order to calculate how much she still needed, you would need to subtract $80 from $360.
Amount needed = $360 - $80.
It tells you that she can earn the rest by washing 20 cars, and asks you to find how much she makes for washing each car. This would represent an unknown, which would be multiplied by 20 to equal the amount needed.
Money earned from washing cars = 20m.
Therefore, this equation can be represented by:
$360 - $80 = 20m.