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9 is the constan trate of change.

Step-by-step explanation:

Assuming two points of the graph with the form as P=(x,y) you can find the constant rate m using the slope formula.  

m=(y2-y1)/(x2-x1)

Lets say that P1=(0,0) and P2=(1,9), then:  

m= (9 – 0)/(1-0)

m=9

Therefore, the point (0,0) means that when you are not buying any t-shirt then you dont have any waste of money. The point (1,9) means that when you are buying one t-shirt then you have to pay 9 then its result that cost of each t-shirt is 9 so when you are buying two t-shirt then you  have to pay 18, when you are buying three you have to pay 27 likewise.