a) Để $(\alpha )//(\beta )<=>\frac{2}{n}=\frac{m}{-8}=\frac{3}{-6}\neq \frac{-5}{2}$

<=> $\left\{\begin{matrix}3.n=3.4 & \\ -6.m=3.(-8) & \end{matrix}\right.<=> \left\{\begin{matrix}n=-4 & \\ m=4 & \end{matrix}\right.$

Vậy $\left\{\begin{matrix}n=-4 & \\ m=4 & \end{matrix}\right.$

b) Để $(\alpha )//(\beta )<=>\frac{3}{2}=\frac{-5}{n}=\frac{m}{-3}\neq \frac{-3}{1}$

<=> $\left\{\begin{matrix}3.n=2.(-5) & \\ 2.m=3.(-3) & \end{matrix}\right.<=> \left\{\begin{matrix}n=\frac{-10}{3} & \\ m=\frac{-9}{2} & \end{matrix}\right.$

Vậy $\left\{\begin{matrix}n=\frac{-10}{3} & \\ m=\frac{-9}{2} & \end{matrix}\right.$