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 :)
p.s. If there is a part of my solution that you did not understand, please do not hesitate to contact me and I'll be more than glad to explain it to you thoroughly
