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A₂₁ = 8 + (21 - 1) (4)

a₂₁ = 8 + (20) (4)

a₂₁ = 8 + 80

a₂₁ = 88

The 21st term is 88.
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Just a tip: If your answer is too short for the minimum characters required, you may write "The answer to your question is...". As I've said. Your other answers, I've checked, are correct and not deleted :-)