the slope equals the rate of change, and is negative in this case
gret
The slope of a line is defined as the ratio of the vertical change (rise) to the horizontal change (run) between any two points on that line. When the rate of change between any two points on a line is negative, it means that the line is sloping downwards from left to right. In terms of slope, this means that the rise is negative and the run is positive, or conversely, the rise is positive and the run is negative. Since the slope is defined as the rise divided by the run, a negative rise divided by a positive run (or vice versa) will always result in a negative slope. So, when the rate of change between any two points on a line is negative, the slope of the line is negative as well.
