Use the ideal gas relation: PV = nRT, and solve for n using 0.082057 L•atm/K•mol for R (since your pressure is given in atm).
From PV = nRT, we get n = PV/RT. Using our given values, we obtain n = (2.3 atm)(120 L)/(0.082057 L•atm/K•mol)(340 K) = 9.9 moles.
The answer is given to two significant figures.