To calculate the center of gravity, divide total weight distance moment by total mass of the system. Thus, the center of gravity is 13 meter from left-hand side.
Assuming it starts at rest, the roller coaster only has potential energy at the top of the hill, which is
When it reaches the bottom, its potential energy will have converted to kinetic energy,
where 
is its velocity at that point. By the law of conservation of energy, assuming no loss of energy due to other sources (e.g. sound, heat), we have
A flow of electric charge from one place to another is electric 'current'.
Also:
When you post a multiple-choice question,
please post the choices along with it.
Thank you.
