We want to calculate the initial concentration in mg/liter, so we need to convert the total amount of gasoline into milligrams and the total volume of water in liters.
First convert the amount of gasoline to milligrams: We had 10 gallons of gasoline spilled and 3.79 liters/gallon means that 37.9 liters of gasoline were spilled. Since the density of gasoline is 800,000 mg/liter, this leaves us with (800,000) (37.9) = 30,320,000 mg of gasoline spilled.
Next convert the volume of water to liters: We know there is 898,800 gallons of water, and 3.79 liters/gallon. So this leaves us with 3,406,452 liters of water.
So this all finally gives us (30,320,000/3,406,452) = 8.9 mg/liter to the nearest tenth.