Answers

2014-05-28T19:45:20+08:00
If all three points are collinear, thus, the slope of every two points is the same all throughout. 

m = -2-6/-3-1 = -8/-4 = 2
m = 6-4/1-a = 2
2/1-a = 2
2 = 2 - 2a
2a =  0
a = 0

Hope this helps :)
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