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m=y2,y1 x2,x1


Boy, you've got me since that's not an equation.

However, if you say that (X1, Y1) and (X2, Y2) are a pair of coordinates, you can get the slope of a line drawn through these two points from the formula

m = \frac {\Delta Y}{\Delta X}= \frac {(Y2-Y1)}{(X2-X1)}

Note that it doesn't matter which point you pick for X1,Y1 and X2,Y2