How do I calculate the distance a car can travel based on a fixed fuel consumption rate and more?

How do I calculate the distance a car can travel based on a fixed fuel consumption rate (in kilometres per litre), the amount of fuel in the tank and the distance already travelled?

Assume that the tank is empty and we add some fuel at the start.

For example, a car has a fixed fuel consumption rate of 6 k/l and I add 3 litres of fuel and I’ve already travelled I don’t know, lets say, 5km.
How do I calculate the quantity of fuel left in the car then after adding the fuel and then travelling over x amount of distance.