What is x when f(x)=1? (this means determine the input that gives an output of 1.)

0 or undefined

Step-by-step explanation:

As you said, it could be anything, and there is proof that could be anything, which I'm sure your teacher would appreciate (writing proofs is a level of mathematical achievement that only hardcore math nerds can do, and it'd be super cool if you could write one yourself) A proof is basically a written mathematical description of exactly why something is the way it is.

But if you want to choose just one answer, I'd choose either 0 or undefined. If the answer is something that's bubbled in, then choose 0.

In the simplest assumed equation f(x) = 1 + x, x would have to = 0.

If we're looking at this graphically, y=1 (remember y=f(x)) graphed is a straight line that never crosses the x axis, which would mean there's no possible x value that would work for that equation.

Also, depending on what level math you're in, you may not even be working with undefined answers yet, in which case, your best answer would again be 0.

Hope this helped!