How do you know what to do to each row in row operations (matrices)??

The price of a visit to the dentist is \$50$50. if the dentist fills any cavities, an additional charge of \$100$100 per cavity gets added to the bill. if the dentist finds nn cavities, what will the cost of the visit be?

What is the common difference in the arithmetic sequence? 7, 14, 21, 28, 2 3 4 7

Can someone me with this math problem? i would greatly appreciate it .