a) 
b) 0.110 megatons
c) 8.46 bombs
Explanation:
a)
The energy lost by the meteorite is equal to the difference between its final kinetic energy and its initial kinetic energy:

Which can be rewritten as:

where:
is the mass of the meteorite
is the final speed of the meteorite
is the initial speed of the meteorite
Substituting the values into the equation, we found the loss in energy of the meteorite:

So, the energy lost by the meteorite is 
b)
The energy equivalent to 1 megaton of TNT is

Here the energy lost by the meteorite is

Therefore, in order to write the energy lost by the meteorite as a multiple of the energy of 1 megaton of TNT, we have to divide the energy lost by the meteorite by the energy equivalent to 1 TNT; we find:

So, the energy lost by the meteorite corresponds to 0.110 megatons.
c)
The energy of one atomic bomb explosion in Hiroshima is equal to
(13 kilotons)
which corresponds to
(0.013 megatons)
Here the energy of the meteorite is equal to
(0.110 megatons)
Therefore, we can find how many Hiroshima bombs are equivalent to teh meteorite impact by using the following rules of three:

So, 8.46 bombs.