Respuesta :

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Answer:

Step-by-step explanation:

The sequence you gave is not arithmetic.  I'm assuming the sequence is

21, 6, -9, -24.

d is the common difference between adjacent terms.

a₂ = a₁ + d

6 = 21 + d

d = -15

nth term = a₁ + (n-1)d = 21 - (n-1)15