It kind of looks like your missing something in you problem. But if f(x) = 3 is correct, then it doesn't matter what x is the answer is always going to be 3. So f(4.5) = 3. Most of the time though f(x) means a function of x, so there is an x in the problem. For example: f(x) = 3x, then f(4.5) = 3*4.5 = 13.5.
