d
\(\left( C _{ v }\right)_{\operatorname{mix}}=\frac{ n _1 Cv _1+ n _2 Cv _2}{ n _1+ n _2}\)

\(\left( C _{ v }\right)_{\operatorname{mix}}=\frac{2 \times \frac{5}{2} R +4 \times \frac{3}{2} R }{2+4}=\frac{11 R }{6}\)

\(\Delta U = n \left( C _{ v }\right)_{\operatorname{mix}} RT =6 \frac{11 R }{6} \times RT =11 R\)