Answer:
Step-by-step explanation:
Remark
This is an arithmetic series. Each term differs by 6 from the previous one.
Givens
a1 = 21
d = 6
Formula
a_n = a1 + (n - 1)*d
Sample
Try it.
Let's let n = 3 which we have the answer for.
a_3 = 21 + (3-1)*6
a_3 = 21 + 2 * 6
a_3 = 21 + 12
a_3 = 33
That's exactly what the 3rd term is.
