Answer
$\Delta U = 360 J $
Work Step by Step
This is a constant pressure process, so the change in internal energy is
$\Delta U = \frac{3}{2} p( V_f - V_i) $
$\Delta U = \frac{3}{2} (4.0 \times 10^4 Pa) ( 8.00 \times 10^{-3} m^3 - 2.00 \times 10^{-3} m^3) $
$\Delta U = 360 J $
