What happens if a balloon during a plane flight in creased from 7 L at 1.0 ATM to 9 L What was the cabin pressure on the airplane during the flight?

This is a job for Boyle's Law!

Mr. Robert Boyle, eminent chemist who died a very long time ago, discovered that for a given mass of gas at a constant temperature, volume and pressure have an inverse relationship. Or to put it another way, if you have a number of grams of gas in a 1-liter bottle at 2000 millibars and you transfer all of it to a 2-liter bottle, the pressure will be reduced to 1000 millibars.

The formula is

P1 x V1 = P2 x V2

At sea level the air pressure is roughly 1000 millibars, and we know we have 7 liters of air are in the balloon. Multiply 7 (the volume) x 1000 (the pressure) and you get 7000. Divide that number by 9 and you've got it.

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Normal commercial flights have a cabin pressure equal to 7,000 ft ASL. That's just enough to prevent passengers from getting altitude sickness, and the lower the cabin pressure the less stress on airframe. Just trivia for what it's worth.