a(n+1)=an+2^n-3na(n+1)-an=2^n-3nan-a(n-1)=2^(n-1)-3(n-1)an - a1 = [2^1+2^2+...+2^(n-1) ] - [ 3+6+...+3(n-1)] = 2^(n-1) -1 - 3n(n-1)/2an = 2^(n-1) +2 - 3n(n-1)/2
