(a)
The velocity of the meteorite just before hitting the ground is:

The loss of energy of the meteorite corresponds to the kinetic energy the meteorite had just before hitting the ground, so:

(b) 1 megaton of tnt is equal to

To find to how many megatons the meteorite energy loss

corresponds, we can set the following proportion

And so we find

So, 0.162 megatons.
(c) 1 Hiroshima bomb is equivalent to 13 kilotons (13 kT). The impact of the meteorite had an energy of

. So, to find to how many hiroshima bombs it corresponds, we can set the following proportion:

And so we find

So, the energy released by the impact of the meteorite corresponds to the energy of 12.46 hiroshima bombs.