Question

what is explicit formula for the sequence of 3,11,19,27.

Answer 1

form a relationship that shows an increase by 8 at a time with a starting point of 3.

Answer 2

8n-5

Answer 3

is it the arithmetic formula?

Answer 4

It will be a relationship with an integer index that increments by "1" for each term. Have you used notation yet that is formed around this symbol?
\[\sum_{ }^{}\]

Answer 5

it is an arithmetic sequence/progression

Answer 6

Your answer will not use the sigma, for that is a summation notation and you will be dealing with a sequence, but I ask because the two are usually introduced at about the same time, series and sequences.