As we know, internal energy is given by, $U=n{C}_{v}T$. Also argon gas is monoatomic and oxygen gas is diatomic.

Therefore, total internal energy will be$U_{net}=n_1{C}_{V_1}T+n_2{C}_{V_2}T$

$=8\times \frac{3R}{2}\times T+6\times \frac{5R}{2}\times T$

$=27RT$