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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