How can one function be put into another function?

If the question asks what f g(x) is, you put one function into another function, to do this find the input (x) Such as x in the equation f (x) = 2x + 1, and then put the second equation of g (x) = x^2 + 1 into the input, so the equation becomes f g (x) = 2 (x^2 + 1) + 1. Now solve and simplify the resultant equation