They're both the same!
(y1 - y2)/(x1 - x2) = -(y2 - y1)/-(x2 - x1)
The negative gets cancelled from the numberator and denominator in the RHS (right hand side) of the above equation.
(y1 - y2)/(x1 - x2) = (y2 - y1)/(x2 - x1)
Do not MIX the order in the numberator and denominator e.g.
(y1 - y2)
(x2 - x1)
Thanks I was thinking that, I just wanted to make sure if there was some instance where it did make a difference I couldn't think up.
"Popular opinion is the greatest lie in the world"-Thomas Carlyle