$1$ mole of $CO$ and $1$ mole of $O _2$ are mixed.

Net internal energy $=\frac{f_1}{2} R T_{ CO }+\frac{f_2}{2} R T_{ O _2}$

$=\frac{5}{2} R 300+\frac{5}{2} R 350$

$=\frac{5}{2} R(650)$

$=5 R(325)$

$=1625 R$

$1625=\frac{5}{2} R T_{\text {final }} \times n_{\text {final }}$

$\frac{1625 \times 2}{5}=T_{\text {final }} \times n_{\text {final }}$

$325 \times 2=T_{\text {final }} \times 2$

$T_{\text {final }}=325 K$

$T_{\text {final }}=37^{\circ} C$