A gas mixture consists of 2 moles of oxygen and 4 moles argon at temperature T. Neglecting all vibrational modes, the total internal energy of the system is

Oxygen is diatomic gas, Hence its energy of 2 moles $2\times \frac{5}{4}.RT=5RT$

Argon is a monoatomic gas. Hence its energy of 4 moles $4\times \frac{3}{2}RT=6RT$

Total internal energy $=5RT+6RT=11RT$