Answer
$m_{AB}\times m_{AC}=-1$
So it is right triangle.
Work Step by Step
We know, that two lines are perpendicular if and only if $m_1m_2=-1$ (as described earlier in this chapter).
Using this we can find whether or not these points correspond to vertices. As the legs of a right triangle are perpendicular to each other, multiplying them should give us $-1$ (We need two, out of three possible sides to be perpendicular). See the image above for a better visualization.
$m_{AB}=\frac{3-(-1)}{3-(-3)}=\frac{4}{6}=\frac{2}{3}$
$m_{BC}=\frac{8-3}{-9-3}=\frac{5}{-12}=-\frac{5}{12}$
$m_{AC}=\frac{8-(-1)}{-9-(-3)}=\frac{9}{-6}=-\frac{3}{2}$
We can clearly see, that: $m_{AB}\times m_{AC}=\frac{2}{3}\times (-\frac{3}{2})=-1$
So, it is right triangle.
