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 ?