arithmetic,
To solve this problem you will need to know the difference between an arithmetic and a geometric.
An arithmetic is a sequence where a person is adding the same number over and over again so lets say you start with 1 and the common difference (the number being added each time) is 2, the sequence will look something like this 1, 3, 5, 7, 9 and so on.
A geometric sequence is when the same number is being multiplied over and over again. So lets say that the number we start with is 2 and you are multiplying by three every single time, so you would get a sequence looking like this 2, 6, 18, 54 and so on.
We can see in the sequence that the number that is being added over and over again is 3 so the first answer is arithmetic.
Now that we know that the sequence is arithmetic and the common difference is 3 we can plug that into the equation and we will see that 13.9 + 3 is 16.9