It doesn't have a negative slope. But s is itself a negative number.
Hey asilvester635.
You have to remember not to get confused by the graph when it implies that a variable is at a particular spot, even when it is not.
Recall the definition of m = (y2 - y1)/(x2 - x1) where y2 = r, x2 = 0, y1 = 0, x1 = s.