If the coefficients of x7 and x8 in 2+x3nare equal, then value of n is

Tr+1, the (r+1)th term in the expansion of  2+x3nis given by                 Tr+1=nCr2n−rx3r=nCr2n−r3rxr  According to the given condition        nC72n−737=nC82n−838⇒    nC7 nC8=2n−838⋅372n−7⇒    n!7!(n−7)!8!(n−8)!n!=16⇒       8n−7=16⇒       48=n−7       ⇒   n=55