How do i show that this fuction is not injective?

Question

The function is given as: $$f: \begin{cases} \mathbb{R} \rightarrow \mathbb{R}^2 \\ t \mapsto (t²-1,t³-t) \end{cases}$$ i know that this is function class $C^\infty$

Answer 1

$f(-1)=(0,0), f(1)=(0,0)$ Hence, the function is not injective.

Answer 2

We have $f(1)=(0,0)=f(-1).$ Conclusion ?