Answer:
This problem can be expressed as a liner model, because it specifies that each week he washes three more cars than the week before, this means that the new week is always three more than the previous week, this implies a sum operation to indicate it.
So, the expression would be:
![y=x+3](https://img.qammunity.org/2019/formulas/mathematics/middle-school/g7344uhwjieao9jzhhqm3pkeqps1d0fb6u.png)
Where
represents the new week, and
represents the previous week.