I know that this formula is correct, but I'm not sure how can I deduce it. Can anyone help me? Thanks in advance!
As it's homework, we do the Socratic method here.
How fast does the capacitor round U8B slew?
When it's going +ve? When it's going -ve?
So how long does it take for each slew?
What voltage is the slew coming from? What voltage is it going to?
How do the values of R1 and R2 affect those voltages?
How does the length o time slewing relate to frequency?
If you've answered all of those questions, then you know more than the question setter. What is the formula in your question missing?