Amy's car holds at most 18 gallons of gas. Now it has 6 gallons. Use pencil and paper. Explain how to find the amount of gas she needs, in liters, to fill the gas tank.

10 gallons

Amy's car's maximum capacity to hold gas = 19 gallons

present amount of gas  = 9 gallons

As the car's capacity is 19 gallons and present amount in tank is 9 gallons, so we can fill it as much as so that the amount in tank becomes 19 gallons.

let assume she fills x gallons

then x gallons and 9 gallons present earlier should be equal to 19 gallon, as the tank can not hold more than that.

lets write it mathematically

x + 9 = 19

subtracting 9 from both side

x + 9 - 9 = 19 - 9

=> x = 10