The rate of change between two points, (x1,y1) and (x2,y2), is equal to the slope of the line connecting the two points. The slope formula is:
slope = (y2-y1) / (x2-x1)
Plug the x and y values from your two points into the formula and compute the answer. It doesn't matter which points you choose for (x1,y1) and (x2,y2); you'll get the same answer.