Answer:
If we double the length we will have:

Step-by-step explanation:
The equation of the angular frequency of a pendulum is given by:

Where:
- g is the gravity
- L is the length of the pendulum
By definition the period is:

And frequency is 1 over the frequency T:


Now, if we double the length we will have:



Therefore, the new frequency is the old frequency over √2.
I hope it helps you!