Two lines are perpendicular if their slopes are negative reciprocals.
The slope of the line between (-2, 1) and (2, -1) is:
(1 - (-1)) / (-2 - 2) = 2 / -4 = -1/2
The slope of one of the other lines is 2. (your job to figure out which one)
Those two lines are negative reciprocals - so they are perpendicular.
So the triangle is a right triangle.