Diameter of the base of the rock = 10800 m
Height = 10200 m
Volume of cone = 1/3πR^2*h = 1245880248190 m^3
It seems that rock was fragmentized (8 j/cc)
E = 9.9e18 Joules (2.3 Gigatons of TNT) Small Island level
Original calc can be found here
Why i disagree with it:
1.Tony didn't need to stop the object (which is when we would have used the KE, even though mommentum is what needs to be opposed) just destroy it.(which is what he did)
2.Using KE just seems as an unreasonable way to get a high result.
First note: His repulsor beams wouldn't do anything to it so if anything this is the peak of his AP.