First, let's find the price of the 4 boxes of cereal. By hypothesis, 2 boxes of cereal cost $4.50. So 4 boxes will cost 2*4.50 = $9.
4 liters of soda costs $8.40. Jasmine bought 2 liters, which is half of the stated amount and price. So the price of 2 liters of soda is 8.40/2 = $4.20
So in total, Jasmine had to pay 9+4.20 = $13.2
She had $20 in her pocket. So she still have 20-13.2 = $6.8
Hope this helps! :)