SOLUTION: http://postimg.org/image/lnl66qwv5/ This is one of the answers I got wrong. Can someone be very specific in how to solve?

Question 1020129: http://postimg.org/image/lnl66qwv5/
This is one of the answers I got wrong. Can someone be very specific in how to solve?
Answer by FrankM(1040) (Show Source): You can put this solution on YOUR website!
Start with this
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.

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