Notice how the y coordinates of points Z and X are both b. It doesn't matter what the x coordinates are. Because both y coordinates are b, this means we have a horizontal line of slope 0.
For points Y and W, the x coordinate is both 'a'. The y coordinate doesn't matter. Since the x coordinate here is both 'a', we have a vertical line with an undefined slope.
ZX is a completely horizontal line
YW is a completely vertical line
Therefore, the diagonals are perpendicular
Note: we can't use the negative reciprocal idea since there is no reciprocal of 0