bn=n4^n
Sn=1*4+2*4^2+...+n*4^n
4Sn=1*4^2+2*4^3+...+(n-1)*4^n+n*4^(n+1)
=>
-3Sn=1*4+4^2+...+4^n - n*4^(n+1)
-3Sn=[1-4^(n+1)]/-3 - n*4^(n+1)
Sn=[1-4^(n+1)]/9 + n*4^(n+1)/3