Explanation:
the slope is (y coordinate change / x coordinate change) when going from one point on the line to another.
the direction does not matter, but it is essential that once a direction is picked for one coordinate, we use the same one then also for the other.
I prefer to go from left to right, so (-5, 0) to (-4, 2) :
x changes by +1 (from -5 to -4).
y changes by +2 (from 0 to 2).
so, the slope is +2/+1 = 2/1 = 2.