There are #6xx6.022xx10^23# atoms in #2.00*mol##NO_2(g)#.
Explanation:
In the one mole of #NO_2#, there are #"Avogadro's number"# of #NO_2# molecules; i.e. #N_A##NO_2# molecules.
And thus in #2.00*mol# of #NO_2# there are #6xxN_A# nitrogen and oxygen atoms. In terms of #N_A#, how many oxygen atoms, and how many nitrogen atoms in this quantity?