arithmetic sequence question please help i have quiz tmrw

[spark dune]

the question asks for the fourth term, then why is the exponent in the red circle 5

[atomic sun]

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[elder crater]

Well idk have you had the binomial theorem yet

The given answer is correct though

[visual cave]

MxtthijsDeLigt

MxtthijsDeLigt

[velvet narwhal]

The formula for binomial theroum is this

This picture should help

If not, I’d be happy to explain