The domain of both of these functions is all real numbers. You can input anything into f(x), and regardless, you will get 3/4. g(x) is a polynomial (specifically a quadratic), which is defined for all real numbers.
The domain of a function

is the set where both f and g are defined. Mathematically, we use ∩ for intersection to show that
∩
. Here, it is just all real numbers for both g and fg.