We know T=$2\pi \sqrt {l/g}$

And here L should be 7r .

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The formula $T=2\pi\sqrt{\frac{L}{g}}$ applies to a simple pendulum - ie a point mass on the end of a massless rod or string. This is not a simple pendulum, it is a compound pendulum.

You need to take into account the moment of inertia $I$ of the ball about the pivot point. You will need to use the parallel axis theorem. The period for a compound pendulum is $2\pi\sqrt{\frac{I}{mgh}}$ where $h$ is the distance of the CM from the pivot.

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