how do you know which data point to put first when calculating the slope?

It doesn't matter....just make sure you subtract the y's and xs in the same order....let me prove this

(6, 4) (2,5)

First way    [ 5 - 4 ] / [ 2 - 6 ]  =  1/-4  = -1/4

Second way  [4 - 5 ] / [ 6 - 2] = -1/4

Note, that I subtracted the y's and the x's in the same  order both times and got the same result...!!!

Ifi had "mixed" the order like this

[5 - 4] / [ 6 - 2]  = 1/4

I would have gotten the wrong answer....!!