13. The line $l$ passes through the points $(-3,0)$ and $(3,-2)$. The line $m$ passes through the points $(1,8)$ and $(-1,2)$. Show that these lines are perpendicular.
Gradient $l$: $-\frac{1}{3}$, gradient $m$: $3$. Product is $-1$, so perpendicular.