A shoebox has dimensions $0\mathrm{.}30\mathrm{.}20\mathrm{.}4$ m$.$ The temperature inside the box is T $=20$C and the pressure is P $=0\mathrm{.}29$ MPa.

Calculate the internal energy in the shoebox given that the specific gas constant, or difference in specific heats, is R $=287$ J kg$-1$ K$-1$ and the specific heat ratio is $=1\mathrm{.}4.$

Give your answer in kJ to one decimal place.