We know that.
Binomial expansion of (2+x)9 is given by,
(2+x)9=C09x029+C19x128+...........C99x920
Now put x=1 we get,
(2+1)9=C0929+C1928++C2927+...........C9920
⇒39=C0929+C1928++C2927+...........C9920
So, sum of coefficient of x,x2,x3,.........,x7 will be,
⇒C1928+C2927+...........C7922=39−C0929−C8921−C9920
⇒C1928+C2927+...........C7922=39−29−18−1
⇒C1928+C2927+...........C7922=19683−512−19
⇒C1928+C2927+...........C7922=19152
Now mean is given by 719152=2736