Question: Roots, lists and substitute

Dear Friends, 

I would appreciate your help in resolving some issues. Let me describe my dummy code and the issues I am having. 

1. I have two functions f(x,a) and g(x,a) are well defined. 

2. I need to get roots of f(x,a). I am using the command soln := Roots(f(x,a)) which gives me a list of all possible values of x. 

3. I need to choose only one element -- say x* -- from the list "soln" such that x*=argmax{g(x,a) | g(x,a)>0 for x in soln}.

I am not able to find a technique (i) to evaluate g(x,a) for all elements of the list "soln", (ii) and select x* that maximizes g(x,a). 

Picking and choosing elements of the list one by one is difficult since the number of elements in the list can vary with parameter "a" and it would complicate the matter in numerical studies. 

I would sincerely appreciate any inputs in this regard. 

Thank you,

Omkar