Given data:
Length of peldulum is,
Period of pendulum is,
New period of pendulum is,
Formula:
Formula of period of pedulum is as follows:
For old period of pendulum above equation becomes as follows:
Taking square of above equation,
Now, for new period of pendulum,
Taking square of above equation,
Hence,
Taking ratio of equation-(3) and equation-(1),
Therefore, Length of pendulum should be 9L for the period to be 3T seconds.