How many nitrogen atoms are in 2.79 mo. of nh3

1 mol of any particles has 6.02 * 10 ²³ particles.

If we look at 1 NH3 (1 mol NH3 or 1 molecule NH3), we can see that 1 molecule NH3 has 1 atom of N and 3 atoms of H; also 1 mole of NH3 has 1 mole of N atoms and 3 moles of H atoms.

So, 1 mol of NH3 has 1 mol of N atoms,
 and 2.79 mol NH3 have 2.79 mol of N atoms.

2.79 mol of N atoms* 6.02 * 10 ²³ N atoms/ 1 mol N atoms = 1.68*10²⁴ N-atoms 

 Answer is 1.68*10²⁴ N-atoms.