finding a closed formula for $\sum_{k=0}^{n} k{2n \choose k}$ my attempt:
$\sum_{k=0}^{n} k{2n \choose k}=\sum_{k=0}^{2n} k{2n \choose k}-\sum_{k=n+1}^{2n} k{2n \choose k}$
the first term in the right hand side
suppose there are  $2n$ poeple, we have to choose a community from them ,and then we have to choose a president from the community chosen ,so if we choose 1 people then there is one possiblity for choose the president ,and if we choose 2 people then there are two possibility for choose the president,and so on,so all ways possible is $\sum_{k=0}^{2n} k{2n \choose k}$,and we can do it by an other way; we can choose $1$ person from them and then we have to choose a community from the remaining $2n-1$;the numbers of ways to do it,is;${2n \choose 1} 2^{2n-1}$
the second term in the right hand side
suppose there are  $2n$ poeple, we have to choose a community from them but thier members doesnot less than  $n+1$ people  ,and then we have to choose a president from the community chosen ,so if we choose $n+1$ people then there is $n+1$ possiblity for choose the president,and so on,so all ways possible is $\sum_{k=n+1}^{2n} k{2n \choose k}$and we can do it by an other way;we can choose $n+1$ people from $2n$ and,and choose a president from those $n+1$ people  then we have to choose a community from the remaining ,that's means;$(n+1){2n \choose n+1} 2^{2n-(n-1)}=(n+1){2n \choose n+1} 2^{n-1}$
so finally;$\sum_{k=0}^{n} k{2n \choose k}=\sum_{k=0}^{2n} k{2n \choose k}-\sum_{k=n+1}^{2n} k{2n \choose k}$
$={2n \choose 1} 2^{2n-1}-(n+1){2n \choose n+1} 2^{n-1}$
does my attempt correct?

Combinatorics are not my strong suite, but I believe the issue lies with your second method for choosing a set of more than $n+1$ from $2n$, where you possibly double count.
You first choose $n+1$ people, then choose a president from this set, then choose additional people to add on. But consider the set $\{a,b,c,d,e,f\}$, choosing $\{a,b,c,f\}$ with $a$ as president and $\{a,b,d,f\}$ with $a$ as president are covered as separate cases in the first two choices. But then they are counted as the same choice if the additional people are chosen as $\{d,e\}$ and $\{c,e\}$ respectively.
Unfortunately, I cannot think of a way to mend your argument to finish the proof. Perhaps someone else can figure something out?
