Here we go, into the mystical land of Mathemagic. Sorry if I goof up at all, let me know and I'll fix it ^.^
The Attack of a Lv 100 Mega Mewtwo X, with 31 IVs, 252 EVs, and an Adamant Nature is 526.
Now, add the Huge Power boost.
526 x 2 = 1052
Next, multiply by 4 for the +6 boost.
1052 x 4 = 4208
Finally, add the Choice Band.
4208 x 1.5 = 6312
Therefore, a Mega Mewtwo X with the specified parameters would have a theoretical Attack stat of a whopping