Explain the difference in solving a function when given the following information: f (x) = 2 and f (2)

F (x) you dont know what x is or f but in f (2) you know that what would be x is 2

so when there is f (x) = 2 without unknowed term of x so this mean that for indifferent value of x the result is 2 for f (2) you need to know f (x) = ? and than just you need substituting the 2 in place of x and calculi the result what will be f (2) = ?