What does (fog) (x) mean?

(fog) (x) means f (g (x)). this means we have to use the two functions f (x) and g (x). In this case, we substitute the function of g (x) to the x - variable of the function of f (x). The two functions can be both polynomials or monomials. In this case, the answer to this problem is f (g (x)).

(fog) (x) = f[g (x) ] ...