7 mole of certain monoatomic ideal gas undergoes a temperature...

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,

$\DeltaU = n{C}_{v}\DeltaT$

$= n \times \dfrac{3R}{2} \times \DeltaT$

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

$\DeltaU = 7 \times \dfrac{3}{2} \times 8 . 3 \times 40 = 3486J$