Answer : $\;11\;RT$

Explanation :

$U= \large\frac{n_{1}f_{1}}{2}$$RT + \large\frac{n_{2}f_{2}}{2} $$RT$

f - No.of degrees of freedom

$f_{1}\;$ for a diatomic gas = 5

$f_{2}\;$ for a monoatomic gas =

Therefore , $\;U=\large\frac{2 \times 5}{2} $$ RT+ \large\frac{4\times 3}{2} $$RT$

$= 11 RT$