Notice that we chose above to start with the second point
Q
and that is why bothdifferences begin with the coordinates of the second point (
y
2
−
y
1
) and (
x
2
−
x
1
).One can start instead with the first point
P
, in which case both differences shouldstart with the coordinates of the first point, namely (
y
1
−
y
2
) and (
x
1
−
x
2
), whichgives the same slope as above