The rate of change (which is basically the slope) can be found by dividing the rise/run or the distance up and sideways from a point on a line to another point.
If we go from the first point (0, -4) to the next point on the line (1, -1), we would go up 3 spaces and right one space. 3/1 which would be 3
I hope this helps, i tried to explain the best i could.
