From the means: $\sum _{i=1}^{10}{x}_i=50$ and $\sum _{i=1}^8{x}_i=32$, so $x_9+x_{10}=18$.

From the variances: $\frac{1}{10}\sum _{i=1}^{10}{x}_i^2-5^2=7$ gives $\sum _{i=1}^{10}{x}_i^2=320$, and $\frac{1}{8}\sum _{i=1}^8{x}_i^2-4^2=3.5$ gives $\sum _{i=1}^8{x}_i^2=156$.

Therefore $x_9^2+x_{10}^2=320-156=164$.

Substituting $x_{10}=18-x_9$: $x_9^2+(18-x_9{)}^2=164$, so $2x_9^2-36x_9+160=0$, i.e. $x_9^2-18x_9+80=0$.

The roots are $x_9=8$ or $x_9=10$. Since $x_9<x_{10}$ and $x_9+x_{10}=18$, we need $x_9<9$, so $x_9=8$ and $x_{10}=10$.

$\therefore 3x_9+2x_{10}=24+20=44$.