if coefficient of x^-7 and x^-8 to the expansion of (2+1/3x)^n are equal then n=?

if coefficient of x^-7 and x^-8 to the expansion of (2+1/3x)^n are equal then n=? If the coefficients of and to the expansion of
are equal then n —
(a) 56
(c) 45
55
• In theexpansion of(l + X)' the coefficient