Answer
2.2R
Work Step by Step
We first find the internal energies of each of the gases:
$E_{intO_{2}}= \frac{5}{2}nRT=5RT$
$E_{int_{Ar}}= \frac{3}{2}nRT=1.5RT$
Adding the internal energies, we find:
$C_v=\frac{6.5RT}{nT}=\frac{6.5R}{3}=2.2R$