I got a theta notation question here as well that im kinda stuck on.

Select a theta notation for each of the following functions, and justify your answers thoroughly.don't really know how to do it and my only initial guesses were a) theta(nlgn) and b) theta(2^n), theta(C^n) c > 1 or theta(C^(n+1)) C > 1

a) root(n) + n + 3nlogn

b) 2 + 4 + 8 + ... + 2^n