59 If three lines: $4 x+y=4, m x+y=0,2 x-3 m y=4$. cannot form a triangle, then the value of $m$ is . $\qquad$
$594,-\frac{1}{6},-1, \frac{2}{3}$. Obviously, when $m=0$, the three known lines can form a triangle. When $m \neq 0$, the slopes of the three lines are $k_{1}=-4, k_{2}=-m, k_{3}=\frac{2}{3 m}$. The three lines cannot form a triangle if any two of them have the same slope. From $k_{1}=k_{2}$ we get $m=4 ; k_{1}=k_{3}$ we get $m=-\frac{1}{6}$; when $k_{2}=k_{3}$ there is no solution. Also, the three lines cannot form a triangle if they all pass through the same point, which happens when $m=-1$ or $\frac{2}{3}$.