What is f (4.5) what si the value of x for which f (x) = 3?

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.