Hello! I know that a function does not have an inverse if it is not a one-to-one function, but I don't know how to prove a function is not one-to-one. Please teach me how to do so using the example below! Thank you!

The function f is defined as f(x) = x^2 -2x -1, x is a real number. Show that the inverse function of f does not exist.

Thanks! :)