Answer:
n = 4
Explanation:
Use the explicit formula of a arthmetic sequence: an = a1 + d(n - 1)
a1 = -70
d = -10
an = -100
Do all of the steps and plug all the numbers with the formula.
So -100 = -70 - 10(n - 1)
-100 = -70 - 10n + 10
-100 = -60 - 10n
-100 + 60 = -40
-40 = -10n
-40/-10n = -10n/10
n = 4
So the final answer is n = 4. Just follow all of the steps and you will understand how I got my answer. Hope it helped!