1) Let's consider the following sequence
34, 25, 16, 7,
a) Each term is decreased by -9 units so the common difference is -9
b) Let's write an explicit formula or equation to represent that Arithmetic Sequence:
a_n = 34 -9(n-1)
c) Finally, with that equation, let's find the 20th term, by plugging into the formula the data we need to find:
a_20 =34 -9(19)
a_20=34 -171
a_20=-137