Answer:
a₁=15 ; aₙ=aₙ₋₁ + 4
Explanation:
In this image, you can see that for each step made, 4 is added to the previous number. a₁ should be 15 because it is the first number seen in the mix. The formula should come out to aₙ=aₙ₋₁+4. This is because the common difference is only 4, and that's essentially all you needed to plug in.