There are an infinite number of possibilities, and not enough information
to decide which possibility is really the one inside the function machine.
Here are a few. Each of these gives the result that you described,
and there are an infinite number of others:
f(x) = x
f(x) = 2x + 1
f(x) = 10x + 9
f(x) = x² - 2
f(x) = 7x² - 8
f(x) = 31x³ + 30
f(x) = log( |x| ) - 1
f(x) = ln( |x| ) - 1
f(x) = x tan(45°)
.
.
etc.
