We apply Dalton's law, where for a gas mixture the total pressure is the sum of the partial pressures of each gas that makes up that mixture. You can add the amount of moles and then calculate the pressure, using the ideal gas formula. We convert the temperature in Celsiud into Kelvin : 0°C= 273K---->
27°C=27 + 273= 300K
PV= (nH+ n0+ nHe)x RT
P=((nH+ n0+ nHe)x RT)/V=
P=((3 mol + 6 mol +9 mol)x 0,082 l atm/ Kmol x300K)/10 L
