Question

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

Answer 1

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.