The first step would be to add 40 to both sides of the equation, you can use multiple methods to solve from there, but I recommend 'completing the square' since your b value is equal to zero :)
Answer: 
Step-by-step explanation:
Do you mean
? In that case just finish things inside the bracket first then multiply 2 inside.

Then multiply 2 inside the bracket.

Move 54 to other side then we get,




(Note: If this is not what you mean then please let me know.)
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.
You can estimate that 5x4257 is 20,000
to check 5x4257=21,375 which is close to 20,000