Question 1020129
Start with this 

{{{m=(y[2]-y[1])/(x[2]-x[1])}}} it's the equation for slope. 

For line ZX,  z is (0,b) x is (2a,b) 

You see how the y values are both b? therefore m=0 as b-b=0 and the 2a on top doesn't much matter. 

For line WY  w is (a,4b) and y is (a,0)

The issue here is that the a-a = 0, and a denominator of 0 produces an "undefined" slope, i.e. a vertical line. 

Perpendicular is defined as the negative reciprocal, therefore the slopes 0 and undef are perpendicular. 

Here is the image. 

*[illustration Screen_shot_2016_02_16_at_7.15.13_PM.png]