Doubt from binomial theorem

Answer for 10

Yes and for question 10 is n(n+1)^2/12 but i got something wrong i got n(n+1)(9n^2-5n+2)/12

Wait a minute I send the solutions

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For 10 I got
\displaystyle \frac{n(n-2)(n+1)^2}{12}

But is written (n+2) maybe answer key is wrong.

Actually now I got
\displaystyle \frac{n(n+2)(n+1)^2}{12}

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In last step it should be \displaystyle n(n+1)^2 \frac{6n+6+3n-8n-4}{12}

I also got i found out where i made mistake.

How to find coefficient of x^r in (1-x)^-n here power is negative how to find it ? I want to solve one question from jan shift 2019 i am stuck here.

Question 17 couldn't understand the solution given in this book.

Please help anyone

As given here is that no. of terms =28
That means n=6

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