I have a function $f(x)$ and I want to prove that it is bijective. The function states
let $f (x) : \mathbb R \rightarrow \mathbb R$ be defined by $f (x) = -x^4$.
I understand that I need to show that it is injective and surjective but I don't understand how I would prove this for a function. I also know the definitions of injection and surjection.