Is the height of the plant proportional to its age?

I found a graph with the same problem, so I guess this is the graph to be based on. To find the rate, just determine the slope between any two points along the line. Suppose these points are: (30,10) and (50,20).

Slope = (20 - 10)/(50 - 30) = 1/2

The correct answer should be: the rate of change is 1 cm per 2 weeks, or 1/2.