Answer
2.20 atm.
Work Step by Step
Use the ideal gas model for nitrogen and for carbon dioxide.
Find the number of moles of nitrogen in 21.6 kg.
$$n_1=\frac{21.6\;kg}{28.0\times10^{-3}kg/mol}=771.4mol$$
Find the number of moles of carbon dioxide in 21.6 kg.
$$n_2=\frac{21.6\;kg}{44.0\times10^{-3}kg/mol}=490.8 mol$$
From the ideal gas law, $\frac{RT}{V}=\frac{P}{n}$ is constant in this situation.
$$\frac{P_1}{n_1}=\frac{P_2}{n_2}$$
$$P_2=P_1\frac{n_2}{n_1}=(3.45\;atm) \frac{490.8}{771.4}=2.20\;atm$$
