Obviously I'm making some sort of systematic error...any help is greatly appreciated!

15.) The number of cars sold weekly by a new auto dealership grows according to a linear growth model. The first week the dealership sold two cars and the second week the dealer ship sold six cars

In the first 50 weeks, how many cars did the dealership sell?

My work:

The correct answer is 5000...

17.) (100 terms)

My work:

The correct answer is 15,350

19.)

Find the sum of the first 100 terms

My work:

Correct Answer: -18,300

Like I said, I'm pretty sure it's a systematic error I'm making...I've tried it lots more way besides this, but I think this is the "most" correct way I've tried....any help will be wonderful. Thanks!!