Step-by-step explanation:
the slope is (y coordinate change / x coordinate change) when going from on point on the line to another.
it does not matter in what direction we go, but it is important to keep the same direction for the x and for the y comparison.
so, because I prefer to go from left to right, let's go from
(-16, 8) to (-3, -4) :
x changes by +13 (from -16 to -3).
y changes by -12 (from 8 to -4).
so, the slope is
-12/+13 = -12/13