Are you sure you haven't posted your formula wrong ?

Because the real formula is :

where

is the length of the pendulum and

.

Anyway, you are given the length of the pendulum, and you know the constant

. Just plug the values into your equation to find

for this pendulum.

So, you find

seconds

Yes I have a lot of figures because I use a high-precision calculator

on your calculator you should have the answer rounded around 10 decimal digits. Thus, you find

seconds.

I do not get the last question.

is irrationnal, so you cannot express

as a fractional number. Do they just mean round it off sensibly ?