The first mistake is that she didn't use parenthesis to indicate we're dividing all of one thing (numerator) over all of another expression (denominator)
slope = rise/run = numerator/denominator
So she should write (17-4)/(1-8) to indicate all of "17-4" is divided over all of "1-8"
However, there's another error that your teacher is probably more focused on. Note how 17-4 represents subtracting the y coordinates from left to right. We start with the left point y coordinate 17 and subtract off the right point y coordinate 4
left y - right y = 17 - 4
But then the student swaps the order when they wrote 1-8. Instead it should be 8-1
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Here's what they should have written (17-4)/(8-1)
This is the same as (4-17)/(1-8)
We can subtract in any order as long as it stays consistent between the numerator and denominator
