Answer
$f=-20cm$
Work Step by Step
Using the relation that $$f=\frac{r}{2}$$ and substituting the value of $r=-40cm$ yields $$f=\frac{-40cm}{2}=-20cm$$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.