Answer:
Since this equation is not complete, I will just give an example.
Explanation:
Lets say Matt starts out with 6 cars in week one. For the next week, just add the number 3 to the preexisting number (6), which will give you 9. Now, lets say he washes cars for 10 weeks to save money. All you would have to do is add a 3 each time to the preexisting number. So, it would go 6, 9, 12, 15, 18, 21, 24, 27 30, 33. The answer to the equation I created above is 33. I hope this helps you solve your question.