internal energy of n moles of a gas is u=1/2nFRT
where F=number of degrees of freedom.
(F=5 for oxygen molecule and F = 3 for Argon)
Internal energy of 2 moles of oxygen at temperature T is;
Internal energy of 4 moles of Argon is:
u2 = 1/2 × 4 × 3RT = 6RT
Total internal energy =u1+u2= 5RT + 6RT = 11RT