The hypotenuse of the two triangles have the same slope because they are in the same straight line and the slope of a straight line is constatn.
You can prove that in this way:
slope = rise / run = Δy / Δx
For the upper triangle: slope = (4 - 2) / (8 - 4) = 2 / 4 = 1 / 2
For the lower triangle: slope = (-1 - (-5)) / (- 2 - (-10)) = 4 / 8 = 1 / 2
So, this proves that the two slopes are the same: 1 / 2
