For the given process pressure is constant therefore, it is an isobaric process.

For a quasi-static process the change in internal energy of an ideal gas is independent of the nature of the process and is given by,

$\Delta U=n{C}_v\Delta T$

$=n\times \frac{3R}{2}\times \Delta T$

[molar heat capacity at constant volume for monatomic gas $=\frac{3R}{2}$]

$\Delta U=7\times \frac{3}{2}\times 8.3\times 40=3486 \mathrm{J}$