In the notation g(x) you know that whatever is put in the parentheses is what replace for x. In this situation, because it says g(b2+1), you replace every x in the function with b2+1. You would then simplify the function further.
If g(x)=3−x5+x,
then g(b2+1)=3−(b2+1)5+(b2+1)=2−b26+b2