3 Collinear Points

[Chaosghoul]

By *FAR* not the easiest method if you want to go quick and dirty.
"A to B X raises 3 and Y raises 2.
So if B to C X raises by six, Y has to increase by 4. It does, so we're collinear."
It's basically also slope formula but imho put in an easier non-formulaic way.
(also, if I'm being an*al about it, your two formulas should read m1 and m2 and in the second formula it's not x1, y1 and x2, y2 but actually x2, y2 and x3, y3 and then you need to show that m1=m2). A strict teacher could deduct quite a few points if you write it like this in a paper