Question 52472
First, you ought to use parentheses to make sure your problem is clearly understood:

(y1-y2)/(x1-x2) and (y2-y1)/(x2-x1)

These are the formulas for finding the slope (m) of a line that passes through two given points. While they may look a little different, both formulas will give you exactly the same result. Let's check using the two points in your problem:
(4, 9) and (3, 6)
{{{(y1-y2)/(x1-x2) = (9-6)/(4-3)}}} = {{{3/1}}} = 3
{{{(y2-y1)/(x2-x1) = (6-9)/(3-4)}}} = {{{(-3)/(-1) = 3/1}}} = 3