We draw the triangle to see what it looks like:
(3, -2), (7, 3), and (-7, 6)
Looks suspiciously like the angle down at (3,-2) is a right
angle. So let's see if we can prove that it is.
We find the slopes of its two sides and if that's a right angle
then the slope of one with be the reciprocal of the other with
the opposite sign
We use the slope formula:
where (x1,y1) = (-7,6)
and where (x2,y2) = (3,-2)
We use the slope formula again:
this time with (x1,y1) = (7,3)
and where (x2,y2) = (3,-2)
Indeed and are reciprocals with
opposite signs, so the sides are perpendicular and
the angle with vertex (3,-2) is a right angle.
So the triangle is a right triangle.
Edwin