The figure is a rectangle.
Slope between two points (x1,y1) and (x2,y2) is (y2-y1)/(x2-x1).
Two lines are perpendicular if the product of their slopes is -1.
Slopes of BG and LH are the same=(4-3)/(-2-(-4))=1/(-2+4)=½,
and (-2-(-3))/(1-(-1))=(-2+3)/(1+1)=½. So BG||LH.
Slopes of GH and BL are the same:
(-2-4)/(1-(-2))=(-6)/3=(-3-3)/(-1-(-4))=-6/(3)=-2. So GH||BL.
Also, (½)(-2)=-1 which means the pairs of parallels are perpendicular. These define a rectangle.