There's no way to calculate the length of the pendulum with the given information.
The only thing we can calculate is the total energy in the pendulum.
When the pendulum is at the bottom of its swing, ALL of its energy is kinetic.
Kinetic energy = (1/2) x (mass) x (speed)²
= (1/2) x (2 kg) x (4.1 m/s)²
= 4.1 kg-m²/sec²
= 4.1 Joules .
In principle, we can also calculate how HIGH the 'bob' is at the end of its swing, because now we know how much total energy the pendulum has, and at the ends of the swing, the energy is all potential.
Potential energy = (mass) x (gravity) x (height)
4.1 J = (2 kg) x (9.8 m/s²) x (height)
Height = (4.1 kg-m²/s²) / (19.6 kg-m/s²)
= 0.209 meter high
and that's as far as we can go. None of this points us toward calculating the length of the pendulum.
If Ig be moment of inertia about an axis through centre of mass and I be moment of inertia through any other axis parallel to earlier axis , then according to theory of parallel axis ,
I = Ig + Md²
where M is mass of the body and d is distance between two parallel axis.
Because kinetic energy do have more than potential energy: kinetic energy is when a object is moving. Potential energy is when something is at rest and has no movement what so ever
