If it is y2-y1/x2-x1 then why can you do y1-y2/x1-x2? Its confusing D:

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You can do both, because a positive on top and a negative on the bottom is the same as a negative on top and a positive on the bottom. For example, 5/-7 is the same as -5/7. The absolute values of two numbers that are subtracted are equal, for example |5-7| is the same as |7-5|

The 1 & 2 are simply references to other things. They have no actual meaning, like an exponent or coefficient. They juat mean the first number and the second number. So, you can put either one first, as long as you go the same way on the top and the bottom. The difference will all come out in the division.