$\Delta {U_{ABC}} = \Delta {U_{AC}}$

$AB$ is isochoric process.

$\Delta {W_{AB}} = 0$

$\Delta {Q_{AB}} = \Delta {U_{AB}} = 400\,J$

$BC$ is isobaric process.

$\Delta {Q_{BC}} = \Delta {U_{BC}} + \Delta {W_{BC}}$

$100 = \Delta {U_{BC}} + 6 \times {10^4}\left( {4 \times {{10}^{ - 3}} - 2 \times {{10}^{ - 3}}} \right)$

$100 = \Delta {U_{BC}} + 12 \times 10$

$\Delta {U_{BC}} = 100 - 120 =  - 20\,J$

$As,\,\Delta {U_{ABC}} = \Delta {U_{AC}}$

$\Delta {U_{AB}} + \Delta {U_{BC}} = \Delta {Q_{AC}} - \Delta {W_{AC}}$

$400 - 20 = \Delta {Q_{AC}} - (2 \times {10^4} \times 2 \times {10^{ - 3}} + \frac{1}{2} \times $

                                                                         $2 \times {10^{ - 3}} \times 4 \times {10^4})$

$\Delta {Q_{AC}} = 460J$