i dont understand how to prove it
if f is a one-to-one fuction, prove that g(x) = -f(x) is also one-to-one
can you show the steps
The function $\displaystyle f$ is one-to-one if and only if $\displaystyle f(x) = f(y) \Rightarrow \;x = y\text{ or }x\ne y \Rightarrow \;f(x) \ne f(y)$
No I will not do this problem for you. That is yours to do.
Just apply the above to $\displaystyle g(x)~.$
"can you show the steps" means that i am trying to understand how to do it for future tests and stop being ignorant, if you don't want to help properly then don't.