Given,
(3+6x)n=3n(1+2x)n
Now if T9 is numerically greatest term, then T8≤T9≥T10
So, C7n3n−7(6x)7≤C8n3n−8(6x)8≥C9n3n−9(6x)9
⇒(n−7)!7!n!9≤(n−8)!8!n!3⋅(6x)≥(n−9)!9!n!(6x)2
⇒⏟(n−7)(n−8)9≤⏟(n−8)818(23)≥9.83649
⇒72≤27(n−7) and 27≥9(n−8)
⇒329≤n and n≤11
So, n0=10
For (3+6x)10
Now Tr+1=Cr10310−r(6x)r
For coefficient of x6
r=6⇒C61034⋅66
For coefficient of x3
r=3⇒C31037⋅63
So, k=C310C610⋅37⋅6334⋅66=6!4!10!10!7!3!⋅8
⇒k=14
∴k+n0=24