When thinking about the rate of change of a graph, it's normal to think of this rate as a fraction of (change in y)/(change in x). If the rate of change is constant, that means the slope of this graph is constant and you are talking about a line. Take the first two points (1,1) and (3,2). The rate of change between these points is (2-1)/(3-1) = 1/2. Try the other ones.