The monthly cost of a family cell phone plan is related to the number of family members who participate in the plan. A collection of data on the number of family members participating in a plan (x) and the cost of the plan (y), in dollars, of 10 families produced the table below.

x y
1 25
1 50
2 30
2 40
3 55
4 40
4 60
5 70
6 80
6 90

Using a residual plot, determine if the following model is a good fit for the data in the table above.

Please help me with this. It's my last question and I can't figure it out.