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Step-by-step explanation:

Givens

a1 = -70

d = -10

an = - 100

Formula

an = a1 + (n - 1)*d

Solution

-100 = -70 + (n - 1)*-10               Remove the brackets.

-100 = - 70 + -10n + 10              Combine

-100 = -60 - 10n                        Add 60 to both sides

-100+60 = -60+60 - 10n           Combine

-40 = - 10n                                 Divide by - 10

-40/-10 = n

n = 4