Scientists are conducting an experiment with a gas in a sealed container. The mass of the gas is measured, and the scientists realize that the gas is leaking over time in a linear way. Nine minutes since the experiment started, the gas had a mass of 68.4 grams. Thirteen minutes since the experiment started, the gas had a mass of 61.2 grams. At what rate is the gas leaking? Use g for grams and min for minutes.