(y - yo) = m.(x - xo)
Let's go
(-1/n - (-1/m)) = s.(1 - n/m)
(-1/n + 1/m) = s.(1 - n/m)
(-m+n)/mn = s.(m - n)/m
If we multiply both sides by m
(-m + n)/n = s.(m - n)
If we take the (-1) from the left side we have
(-1.(m - n))/n = s.(m - n)
And if we divide both sides by (m - n)
-1/n = s
So our slope is -1/n
