$C_nH_{2n}+H_2\rightarrow C_nH_{2n+2}$$\begin{cases}n_{C_nH_{2n}}=x mol \\ n_{H_2}=y mol\Rightarrow \begin{cases}\frac{14nx+2y}{x+y}=2.9,1 \\ \frac{14nx+2y}{y}=2.13 \end{cases} \end{cases}\Rightarrow \frac{y}{x+y}=\frac{9,1}{13}\Rightarrow \frac{y}{x}=\frac{9,1}{3,9}$
$\Rightarrow \frac{14nx+\frac{2.9,1x}{3,9}}{\frac{9,1x}{3,9}}=26\Rightarrow n=4\Rightarrow CH_3-CH=CH-CH_3$