Question

sqrt(x+9)

Answer 1

Well, we could argue about if the square root can be negative. However the way I see it for x = 0 for example, f(x) could be either -3 or +3. A function can have only one value of f(x) for each value of x so I say this is not a function.
By the way the inverse, y = x^2-9 is a function.