You did not include the graph.
Searching on internet I found a graph with which I can help you. I will solve the question using and so you can understand the problem and solve with the real graph.
1) The slope of line given the graph is calculated as:
slope = run / rise = Δy / Δx = constant.
2) So, you can choose any two points to calculate the slope.
3) Uisng the points (50,20) and (30,10) you get:
slope = [ 20 - 10] / [50 - 30] = 10 / 20 = 1/2
4) Note that that means that the plant grows 1 cm in 2 days
5) The rate of change is 1cm / 2 weeks.
With this you know how to calculate the slope, which is the same rate of change, and the meaning of it, which permits to answer the question from the complete statement and graph.